diff --git a/src/main/java/de/vanitasvitae/imi/codes/persistence/FileRepository.java b/src/main/java/de/vanitasvitae/imi/codes/persistence/FileRepository.java
index 982693e..e0dc588 100644
--- a/src/main/java/de/vanitasvitae/imi/codes/persistence/FileRepository.java
+++ b/src/main/java/de/vanitasvitae/imi/codes/persistence/FileRepository.java
@@ -12,6 +12,16 @@ import java.util.HashMap;
 import de.vanitasvitae.imi.codes.input.InvalidOptionException;
 import de.vanitasvitae.imi.codes.types.StudyNumber;
 
+/**
+ * Repository, that stores generated sample codes per study to files on disk.
+ * The repository stores the next sample code which would be generated for any {@link StudyNumber} {@code stn} to
+ * the directory {@code <base>/stn}.
+ * If no sample number record has been found for a {@link StudyNumber}, 0 is returned.
+ *
+ * Additionally, the repository is initialized with some hard-coded values for studies "AAA" and "BBB".
+ * If the user tries to generate codes for those studies, the generated codes start at the hard coded values instead
+ * of 0.
+ */
 public class FileRepository {
 
     private final File base;
@@ -30,10 +40,20 @@ public class FileRepository {
             hardCoded.put(new StudyNumber("AAA"), 35);
             hardCoded.put(new StudyNumber("BBB"), 42);
         } catch (InvalidOptionException e) {
+            // It should never happen here, that the hard coded study numbers are invalid.
             throw new AssertionError(e);
         }
     }
 
+    /**
+     * Return the base directory of the repository.
+     * @return base directory
+     */
+    public File getBaseDirectory() {
+        return base;
+    }
+
+    // Returns a hard coded value, or 0 if absent.
     private int getHardCodedOrZero(StudyNumber studyNumber) {
         Integer i = hardCoded.get(studyNumber);
         return (i != null ? i : 0);
diff --git a/src/test/java/de/vanitasvitae/imi/codes/InputValidatorTest.java b/src/test/java/de/vanitasvitae/imi/codes/input/InputValidatorTest.java
similarity index 99%
rename from src/test/java/de/vanitasvitae/imi/codes/InputValidatorTest.java
rename to src/test/java/de/vanitasvitae/imi/codes/input/InputValidatorTest.java
index 790adfa..3629723 100644
--- a/src/test/java/de/vanitasvitae/imi/codes/InputValidatorTest.java
+++ b/src/test/java/de/vanitasvitae/imi/codes/input/InputValidatorTest.java
@@ -1,4 +1,4 @@
-package de.vanitasvitae.imi.codes;
+package de.vanitasvitae.imi.codes.input;
 
 import static junit.framework.TestCase.assertEquals;
 
diff --git a/src/test/java/de/vanitasvitae/imi/codes/persistence/FileRepositoryTest.java b/src/test/java/de/vanitasvitae/imi/codes/persistence/FileRepositoryTest.java
new file mode 100644
index 0000000..6e3b4e3
--- /dev/null
+++ b/src/test/java/de/vanitasvitae/imi/codes/persistence/FileRepositoryTest.java
@@ -0,0 +1,90 @@
+package de.vanitasvitae.imi.codes.persistence;
+
+import static junit.framework.TestCase.assertEquals;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.Stack;
+
+import de.vanitasvitae.imi.codes.input.InvalidOptionException;
+import de.vanitasvitae.imi.codes.persistence.FileRepository;
+import de.vanitasvitae.imi.codes.types.StudyNumber;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class FileRepositoryTest {
+
+    private static FileRepository repository;
+
+    @BeforeClass
+    public static void setup() {
+        // Create a temporary test directory in %TEMP%/imicodes
+        File systemTemp = new File(System.getProperty("java.io.tmpdir"));
+        File tempDir = new File(systemTemp, "imicodes");
+        repository = new FileRepository(tempDir);
+    }
+
+    /**
+     * Test, if the {@link FileRepository} correctly stores and returns values to/from file, and if hard coded values
+     * are respected the first time they are used.
+     *
+     * @throws InvalidOptionException NOT expected.
+     * @throws IOException hopefully NOT expected (the temp dir should be readable)
+     */
+    @Test
+    public void testReadWriteSampleNumbers() throws InvalidOptionException, IOException {
+        StudyNumber studyNumber = new StudyNumber("abc");
+        assertEquals("An unknown study number must return 0 as next sample number",
+                0, repository.nextSampleCode(studyNumber));
+
+        // The written next sample code must be returned next time
+        int next = 34;
+        repository.writeNextSampleCode(studyNumber, next);
+        assertEquals("The written next sample number must be read next time.",
+                next, repository.nextSampleCode(studyNumber));
+
+        // The FileRepository contains 35 as hard coded sample number for study "AAA".
+
+        StudyNumber hardCoded = new StudyNumber("AAA");
+        assertEquals("The hard coded value must be returned the first time its being read.",
+                35, repository.nextSampleCode(hardCoded));
+        repository.writeNextSampleCode(hardCoded, 89);
+        assertEquals("If the hard coded value is overwritten, the new value must be read.",
+                89, repository.nextSampleCode(hardCoded));
+    }
+
+    @AfterClass
+    public static void tearDown() {
+        // Delete the temporary directory which was set up earlier.
+        deleteDirectory(repository.getBaseDirectory());
+    }
+
+    /**
+     * Recursively delete a directory and its contents.
+     *
+     * @param root root directory
+     */
+    private static void deleteDirectory(File root) {
+        if (!root.exists()) {
+            return;
+        }
+        File[] currList;
+        Stack<File> stack = new Stack<>();
+        stack.push(root);
+        while (!stack.isEmpty()) {
+            if (stack.lastElement().isDirectory()) {
+                currList = stack.lastElement().listFiles();
+                if (currList != null && currList.length > 0) {
+                    for (File curr : currList) {
+                        stack.push(curr);
+                    }
+                } else {
+                    stack.pop().delete();
+                }
+            } else {
+                stack.pop().delete();
+            }
+        }
+    }
+}