diff --git a/include/caosdb/connection.h b/include/caosdb/connection.h
index 22bdc6f9f5b524ae895469d25d540f9679e65353..5d7930a0d63e484f01be52dab7749d02b3916c33 100644
--- a/include/caosdb/connection.h
+++ b/include/caosdb/connection.h
@@ -149,6 +149,9 @@ public:
     return ConnectionManager::GetInstance().mGetConnection(name);
   };
 
+  /**
+   * Get the connection marked by the "default" key in the configuration.
+   */
   inline static auto GetDefaultConnection()
     -> const std::shared_ptr<Connection> & {
     return ConnectionManager::GetInstance().mGetDefaultConnection();
diff --git a/include/caosdb/entity.h b/include/caosdb/entity.h
index 4803ae7ca7de2d9dfad688ed4a586e11418e7ad9..e4f628ecede26653880d2bcaeb7e94c27cf33de1 100644
--- a/include/caosdb/entity.h
+++ b/include/caosdb/entity.h
@@ -111,6 +111,12 @@ private:
   friend class Entity;
 };
 
+/**
+ * Messages convey information about the state and result of transactions.
+ *
+ * A Message object can be thought of as kinf of a generalized error object in other frameworks.
+ * Please have a look at MessageCodes for more details.
+ */
 class Message {
 public:
   explicit inline Message(caosdb::entity::v1alpha1::Message *wrapped)
diff --git a/include/caosdb/log_level.h b/include/caosdb/log_level.h
index af05d3ba11b3d0e6252c889f7c9be1ddd61015f8..c2fcb9b1181dcb86f91dfaf9a4ddecba3ea417cb 100644
--- a/include/caosdb/log_level.h
+++ b/include/caosdb/log_level.h
@@ -19,8 +19,8 @@
  *
  */
 
-#ifndef CAOSDB_LOG_LEVELS_H
-#define CAOSDB_LOG_LEVELS_H
+#ifndef CAOSDB_LOG_LEVEL_H
+#define CAOSDB_LOG_LEVEL_H
 
 #define CAOSDB_LOG_LEVEL_OFF 1000000
 #define CAOSDB_LOG_LEVEL_FATAL 700
diff --git a/include/caosdb/logging.h b/include/caosdb/logging.h
index 26ec1c8465b6805132f66853cea5aa4e384018ed..d111f784b25468b68784f347a7d4cc067939f6d2 100644
--- a/include/caosdb/logging.h
+++ b/include/caosdb/logging.h
@@ -43,6 +43,9 @@ typedef boost::log::sources::severity_channel_logger<int, std::string>
 
 BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT(logger, boost_logger_class)
 
+/**
+ * This class stores the integer log level.
+ */
 class LevelConfiguration {
 public:
   LevelConfiguration(int level) : level(level){};
@@ -54,6 +57,11 @@ private:
 
 class SinkConfiguration;
 
+/**
+ * This class stores the logging level and log sinks.
+ *
+ * Sinks are represented by SinkConfiguration objects.
+ */
 class LoggingConfiguration : public LevelConfiguration {
 public:
   virtual ~LoggingConfiguration() = default;
@@ -69,6 +77,22 @@ private:
 auto initialize_logging_defaults() -> int;
 auto initialize_logging(const LoggingConfiguration &configuration) -> void;
 
+/**
+ * A logging sink is characterized by a name and destination.
+ *
+ * Typical inheriting configurations exist for console, files and syslog.
+ *
+ * When a SinkConfiguration is created from a configuration, the sink configuration must contain a
+ * \p destination key which matches one of the keywords for implemented sinks.  At the moment of
+ * writing this documentation, valid destinations are:
+ *
+ * \li \p file
+ * \li \p console
+ * \li \p syslog
+ *
+ * A \p level keyword sets the logging level, if it exists at the sink or logging level of the
+ * configuration.
+ */
 class SinkConfiguration : public LevelConfiguration {
 public:
   virtual ~SinkConfiguration() = default;
@@ -104,6 +128,12 @@ private:
   const std::string destination = "Console";
 };
 
+/**
+ * The file name is the destination, the directory can be set separately.
+ *
+ * If there is a `directory` key in the configuration, that will be used as a default, otherwise it
+ * is the current directory.
+ */
 class FileSinkConfiguration : public SinkConfiguration {
 public:
   virtual ~FileSinkConfiguration() = default;
@@ -136,27 +166,27 @@ private:
 };
 
 /**
- * Convenience function for the c-interface.
+ * Convenience function for the C interface.
  */
 void caosdb_log_fatal(const char *channel, const char *msg);
 /**
- * Convenience function for the c-interface.
+ * Convenience function for the C interface.
  */
 void caosdb_log_error(const char *channel, const char *msg);
 /**
- * Convenience function for the c-interface.
+ * Convenience function for the C interface.
  */
 void caosdb_log_warn(const char *channel, const char *msg);
 /**
- * Convenience function for the c-interface.
+ * Convenience function for the C interface.
  */
 void caosdb_log_info(const char *channel, const char *msg);
 /**
- * Convenience function for the c-interface.
+ * Convenience function for the C interface.
  */
 void caosdb_log_debug(const char *channel, const char *msg);
 /**
- * Convenience function for the c-interface.
+ * Convenience function for the C interface.
  */
 void caosdb_log_trace(const char *channel, const char *msg);
 
diff --git a/include/caosdb/message_code.h b/include/caosdb/message_code.h
index a5b65ca68cae8938abb2929888b8201d993e706b..ca08edf32e01d8535c77e1a3a82b45914159cfd4 100644
--- a/include/caosdb/message_code.h
+++ b/include/caosdb/message_code.h
@@ -30,6 +30,9 @@
  * In contrast to the status codes, the message codes are part of the CaosDB
  * API. Messages (and their codes) represent the state of the entities in a
  * transaction or the server.
+ *
+ * For a specification of the message codes, look at the protobuf documentation.  The sources and
+ * documentation can be found at https://gitlab.indiscale.com/caosdb/src/caosdb-proto.
  */
 
 namespace caosdb::entity {
diff --git a/include/caosdb/transaction_status.h b/include/caosdb/transaction_status.h
index ff5029c760af55a36a0f8ae1473300ad3c1bcae5..32465c61e80c4f0f197a54b9c867ea1eb13801e1 100644
--- a/include/caosdb/transaction_status.h
+++ b/include/caosdb/transaction_status.h
@@ -25,7 +25,11 @@
 /**
  * TransactionStatus indicates the current status of a transaction and, when it
  * has already terminated, whether the transaction has been successful or not.
+ *
+ * A status code of 0 denotes a generic success state, positive values indicate errors, and negative
+ * values indicate other states, such as different stages of a transaction in process.
  */
+
 #include "caosdb/status_code.h"
 #include "caosdb/exceptions.h"
 #include <memory> // for shared_ptr, unique_ptr
@@ -130,7 +134,7 @@ public:
   /**
    * Return a description of the erroneous state.
    *
-   * Returns an empty string if there is no description.
+   * No description yields an empty string.
    */
   inline auto GetDescription() const -> const std::string & {
     return this->description;
diff --git a/include/caosdb/utility.h b/include/caosdb/utility.h
index e1e306e40a82ced49ccefea45a42a9e63a2b04b9..fbfc5c181755bc08993b1c50c2baad30269c4d6b 100644
--- a/include/caosdb/utility.h
+++ b/include/caosdb/utility.h
@@ -43,8 +43,6 @@ using boost::json::value;
 
 /**
  * @brief Read a text file into a string and return the file's content.
- *
- * TODO use boost-filesystem's "load_string_file"!
  */
 inline auto load_string_file(const path &path) -> std::string {
   std::string result;
@@ -52,11 +50,14 @@ inline auto load_string_file(const path &path) -> std::string {
   return result;
 }
 
-inline auto get_env_var(const char *key, const char *fall_back) -> const
+/**
+ * @brief Return the environment variable KEY, or FALLBACK if it does not exist.
+ */
+inline auto get_env_var(const char *key, const char *fallback) -> const
   char * {
   const char *val = getenv(key);
   if (val == nullptr) {
-    return fall_back;
+    return fallback;
   } else {
     return val;
   }
@@ -64,11 +65,11 @@ inline auto get_env_var(const char *key, const char *fall_back) -> const
 
 /**
  * @brief Return the value of an environment variable or - if undefined - the
- * fall_back value.
+ * fallback value.
  */
-inline auto get_env_var(const std::string &key, const std::string &fall_back)
+inline auto get_env_var(const std::string &key, const std::string &fallback)
   -> const std::string {
-  const char *val = get_env_var(key.c_str(), fall_back.c_str());
+  const char *val = get_env_var(key.c_str(), fallback.c_str());
 
   auto const result = std::string(val);
   return result;
diff --git a/include/ccaosdb.h b/include/ccaosdb.h
index 3d57a019556d0d1a3edf50d0106d9f463e2a2693..957e2fb7fc5d865d23cfcabdf5e078d270b5ba35 100644
--- a/include/ccaosdb.h
+++ b/include/ccaosdb.h
@@ -81,9 +81,9 @@ typedef struct {
 /**
  * Return the environment variable of the given name.
  *
- * If the environment variable is not set, return the fall_back instead.
+ * If the environment variable is not set, return the fallback instead.
  */
-const char *caosdb_utility_get_env_var(const char *name, const char *fall_back);
+const char *caosdb_utility_get_env_var(const char *name, const char *fallback);
 
 /**
  * Return a description of the status code.
diff --git a/src/caosdb/configuration.cpp b/src/caosdb/configuration.cpp
index 4a1f2a63f58274db3958e71c983f01298290cabf..618c212912ffb65b073d4a0685a89ceb7ad386c6 100644
--- a/src/caosdb/configuration.cpp
+++ b/src/caosdb/configuration.cpp
@@ -508,6 +508,7 @@ auto ConfigurationManager::InitializeDefaults() -> int {
     mLoadSingleJSONConfiguration(*configuration_file_path);
   }
 
+  // Logging in the configuration leads to additional content.
   if (this->json_configuration.is_object() &&
       this->json_configuration.as_object().contains("logging")) {
     LoggingConfiguration logging_configuration =
diff --git a/src/caosdb/logging.cpp b/src/caosdb/logging.cpp
index 486ab846c4ce43225b9242aceb557775b9632531..3618826a5ed6061f210c393b3429b641293fccae 100644
--- a/src/caosdb/logging.cpp
+++ b/src/caosdb/logging.cpp
@@ -59,6 +59,7 @@ auto LoggingConfiguration::GetSinks() const
 
 SinkConfiguration::SinkConfiguration(std::string name, int level)
   : LevelConfiguration(level), name(std::move(name)) {}
+
 [[nodiscard]] auto SinkConfiguration::GetName() const -> const std::string & {
   return this->name;
 }
@@ -77,6 +78,7 @@ auto SinkConfiguration::Configure(boost::log::settings &settings) const
 ConsoleSinkConfiguration::ConsoleSinkConfiguration(const std::string &name,
                                                    int level)
   : SinkConfiguration(name, level) {}
+
 [[nodiscard]] auto ConsoleSinkConfiguration::GetDestination() const
   -> const std::string & {
   CAOSDB_LOG_TRACE(logger_name)
@@ -93,12 +95,14 @@ auto ConsoleSinkConfiguration::Configure(boost::log::settings &settings) const
 
 FileSinkConfiguration::FileSinkConfiguration(const std::string &name, int level)
   : SinkConfiguration(name, level) {}
+
 [[nodiscard]] auto FileSinkConfiguration::GetDestination() const
   -> const std::string & {
   CAOSDB_LOG_TRACE(logger_name)
     << "Enter FileSinkConfiguration::GetDestination()";
   return this->destination;
 }
+
 auto FileSinkConfiguration::SetDirectory(const std::string &directory) -> void {
   this->directory = std::string(directory);
 }
@@ -114,11 +118,13 @@ auto FileSinkConfiguration::Configure(boost::log::settings &settings) const
 SyslogSinkConfiguration::SyslogSinkConfiguration(const std::string &name,
                                                  int level)
   : SinkConfiguration(name, level) {}
+
 [[nodiscard]] auto SyslogSinkConfiguration::GetDestination() const
   -> const std::string & {
   return this->destination;
 }
 
+// Called if no custom logging settings are specified.
 auto initialize_logging_defaults() -> int {
   // first: turn everything off
   boost::log::settings off_settings;
@@ -148,6 +154,7 @@ auto initialize_logging_defaults() -> int {
   return 0;
 }
 
+// Called if custom logging settings are specified.
 auto initialize_logging(const LoggingConfiguration &configuration) -> void {
   boost::log::settings new_settings;
 
diff --git a/src/ccaosdb.cpp b/src/ccaosdb.cpp
index d21b3bd0aafd9c95488658574ef6c03a044996c1..4ed9188f2973f2056ca8cacf7a9d86edd878b5f8 100644
--- a/src/ccaosdb.cpp
+++ b/src/ccaosdb.cpp
@@ -57,8 +57,8 @@ const char *caosdb_constants_COMPATIBLE_SERVER_VERSION_PRE_RELEASE() {
 }
 
 const char *caosdb_utility_get_env_var(const char *name,
-                                       const char *fall_back) {
-  return caosdb::utility::get_env_var(name, fall_back);
+                                       const char *fallback) {
+  return caosdb::utility::get_env_var(name, fallback);
 }
 
 const char *caosdb_get_status_description(int code) {