diff --git a/code/DMSDocument.php b/code/DMSDocument.php
index e63b867..ef95cb8 100644
--- a/code/DMSDocument.php
+++ b/code/DMSDocument.php
@@ -6,6 +6,7 @@ class DMSDocument extends DataObject implements DMSDocumentInterface {
 		"Folder" => "Varchar(255)",	// eg.	0
 		"Title" => 'Varchar(1024)', // eg. "Energy Saving Report for Year 2011, New Zealand LandCorp"
 		"Description" => 'Text',
+		"LastChanged" => 'DateTime' //when this document was created or last replaced (small changes like updating title don't count)
 	);
 
 	static $many_many = array(
@@ -70,28 +71,51 @@ class DMSDocument extends DataObject implements DMSDocumentInterface {
 				$tag = new DMSTag();
 				$tag->Category = $category;
 				$tag->Value = $value;
+				$tag->MultiValue = true;
 				$tag->write();
 				$tag->Documents()->add($this);
+			} else {
+				//add the relation between the tag and document
+				foreach($currentTag as $tagObj) {
+					$tagObj->Documents()->add($this);
+				}
 			}
 		} else {
 			//single value tag
 			$currentTag = $this->Tags()->filter(array('Category' => $category));
-			if (!$currentTag) {
+			$tag = null;
+			if ($currentTag->Count() == 0) {
 				//create the single-value tag
 				$tag = new DMSTag();
 				$tag->Category = $category;
 				$tag->Value = $value;
+				$tag->MultiValue = false;
 				$tag->write();
-				$tag->Documents()->add($this);
 			} else {
 				//update the single value tag
 				$tag = $currentTag->first();
 				$tag->Value = $value;
+				$tag->MultiValue = false;
 				$tag->write();
 			}
+
+			//regardless of whether we created a new tag or are just updating an existing one, add the relation
+			$tag->Documents()->add($this);
 		}
 	}
 
+	protected function getTagsObjects($category, $value = null) {
+		$valueFilter = array("Category" => $category);
+		if (!empty($value)) $valueFilter['Value'] = $value;
+
+		if ($this->ID == 2) {
+		Debug::Show($this);
+		Debug::Show($this->Tags());
+		}
+		$tags = $this->Tags()->filter($valueFilter);
+		return $tags;
+	}
+
 	/**
 	 * Fetches all tags associated with this DMSDocument within a given category. If a value is specified this method
 	 * tries to fetch that specific tag.
@@ -101,16 +125,13 @@ class DMSDocument extends DataObject implements DMSDocumentInterface {
 	 * @return array of Strings of all the tags or null if there is no match found
 	 */
 	function getTags($category, $value = null) {
-		$valueFilter = array("Category" => $category);
-		if (!empty($value)) $valueFilter['Value'] = $value;
-
-		$tag = $this->Tags()->filter($valueFilter);
+		$tags = $this->getTagsObjects($category, $value);
 
 		//convert DataList into array of Values
 		$returnArray = null;
-		if ($tag->Count() > 0) {
+		if ($tags->Count() > 0) {
 			$returnArray = array();
-			foreach($tag as $t) {
+			foreach($tags as $t) {
 				$returnArray[] = $t->Value;
 			}
 		}
@@ -141,7 +162,27 @@ class DMSDocument extends DataObject implements DMSDocumentInterface {
 	 * @return null
 	 */
 	function removeTag($category, $value = null) {
-		// TODO: Implement removeTag() method.
+		$tags = $this->getTagsObjects($category, $value);
+
+		if ($tags->Count() > 0) {
+			$tagsToDelete = array();
+
+			foreach($tags as $t) {
+				$documentList = $t->Documents();
+
+				//remove the relation between the tag and the document
+				$documentList->remove($this);
+
+				//delete the entire tag if it has no relations left
+				if ($documentList->Count() == 0) $tagsToDelete[] = $t->ID;
+			}
+
+			//delete after the loop, so it doesn't conflict with the loop of the $tags list
+			foreach($tagsToDelete as $tID) {
+				$tag = DataObject::get_by_id("DMSTag",$tID);
+				$tag->delete();
+			}
+		}
 	}
 
 	/**
@@ -149,7 +190,10 @@ class DMSDocument extends DataObject implements DMSDocumentInterface {
 	 * @return null
 	 */
 	function removeAllTags() {
-		// TODO: Implement removeAllTags() method.
+		$allTags = $this->Tags();
+		foreach($allTags as $tag) {
+			if ($tag->Documents()->Count() == 0) $tag->delete();
+		}
 	}
 
 	/**
@@ -278,4 +322,14 @@ class DMSDocument extends DataObject implements DMSDocumentInterface {
 		parent::delete();
 	}
 
+	/**
+	 * Takes a File object or a String (path to a file) and copies it into the DMS, replacing the original document file
+	 * but keeping the rest of the document unchanged.
+	 * @param $file File object, or String that is path to a file to store
+	 * @return DMSDocumentInstance Document object that we replaced the file in
+	 */
+	function replaceDocument($file) {
+		// TODO: Implement replace() method.
+	}
+
 }
\ No newline at end of file
diff --git a/code/DMSTag.php b/code/DMSTag.php
index 9387add..6cf2c14 100644
--- a/code/DMSTag.php
+++ b/code/DMSTag.php
@@ -5,8 +5,9 @@
 class DMSTag extends DataObject {
 
 	static $db = array(
-		'Category' => 'varchar(1024)',
-		'Value' => 'varchar(1024)'
+		'Category' => 'Varchar(1024)',
+		'Value' => 'Varchar(1024)',
+		'MultiValue' => 'Boolean(1)'
 	);
 
 	static $belongs_many_many = array(
diff --git a/code/interface/DMSDocumentInterface.php b/code/interface/DMSDocumentInterface.php
index 702e855..3c1c9ce 100644
--- a/code/interface/DMSDocumentInterface.php
+++ b/code/interface/DMSDocumentInterface.php
@@ -123,6 +123,14 @@ interface DMSDocumentInterface {
 	 */
 	function downloadLink();
 
+	/**
+	 * Takes a File object or a String (path to a file) and copies it into the DMS, replacing the original document file
+	 * but keeping the rest of the document unchanged.
+	 * @param $file File object, or String that is path to a file to store
+	 * @return DMSDocumentInstance Document object that we replaced the file in
+	 */
+	function replaceDocument($file);
+
 	/**
 	 * Hides the DMSDocument, so it does not show up when getByPage($myPage) is called
 	 * (without specifying the $showEmbargoed = true parameter). This is similar to expire, except that this method
diff --git a/tests/DMSTagTest.php b/tests/DMSTagTest.php
index 34fc62c..eb927c8 100644
--- a/tests/DMSTagTest.php
+++ b/tests/DMSTagTest.php
@@ -4,7 +4,6 @@ class DMSTagTest extends SapphireTest {
 	//static $fixture_file = "dms/tests/dmstest.yml";
 
 	function testAddingTags() {
-
 		$doc = new DMSDocument();
 		$doc->Filename = "test file";
 		$doc->Folder = "0";
@@ -20,5 +19,87 @@ class DMSTagTest extends SapphireTest {
 		$this->assertNotNull($fruits,"Something returned for fruit tags");
 		$this->assertEquals(count($fruits),3,"3 fruit tags returned");
 		$this->assertTrue(in_array("banana",$fruits),"correct fruit tags returned");
+
+		//sneakily create another document and link one of the tags to that, too
+		$doc2 = new DMSDocument();
+		$doc2->Filename = "sneaky file";
+		$doc2->Folder = "0";
+		$doc2->write();
+		$doc2->addTag("fruit","banana");
+
+		$fruits = $doc->getTags("fruit");
+		$this->assertNotNull($fruits,"Something returned for fruit tags");
+		$this->assertEquals(count($fruits),1,"Only 1 fruit tags returned");
+
+		//tidy up by deleting all tags from doc 1 (But the banana fruit tag should remain)
+		$doc->removeAllTags();
+
+		//banana fruit remains
+		$doc2->getTags("fruit");
+		$this->assertNotNull($fruits,"Something returned for fruit tags");
+		$this->assertEquals(count($fruits),1,"Only 1 fruit tags returned");
+
+		$tags = DataObject::get("DMSTag");
+		$this->assertEquals($tags->Count(),1,"A single DMS tag objects remain after deletion of all tags on doc1");
+
+		//delete all tags off doc2 to complete the tidy up
+		$doc2->removeAllTags();
+
+		$tags = DataObject::get("DMSTag");
+		$this->assertEquals($tags->Count(),0,"No DMS tag objects remain after deletion");
 	}
+
+	function testRemovingTags() {
+		$doc = new DMSDocument();
+		$doc->Filename = "test file";
+		$doc->Folder = "0";
+		$doc->write();
+
+		$doc->addTag("fruit","banana");
+		$doc->addTag("fruit","orange");
+		$doc->addTag("fruit","apple");
+		$doc->addTag("company","apple");
+		$doc->addTag("company","SilverStripe");
+
+		$companies = $doc->getTags("company");
+		$this->assertNotNull($companies,"Companies returned before deletion");
+		$this->assertEquals(count($companies),2,"Two companies returned before deletion");
+
+		//delete an entire category
+		$doc->removeTag("company");
+
+		$companies = $doc->getTags("company");
+		$this->assertNull($companies,"All companies deleted");
+
+		$fruit = $doc->getTags("fruit");
+		$this->assertEquals(count($fruit),3,"Three fruits returned before deletion");
+
+		//delete a single tag
+		$doc->removeTag("fruit","apple");
+
+		$fruit = $doc->getTags("fruit");
+		$this->assertEquals(count($fruit),2,"Two fruits returned after deleting one");
+
+		//delete a single tag
+		$doc->removeTag("fruit","orange");
+
+		$fruit = $doc->getTags("fruit");
+		$this->assertEquals(count($fruit),1,"One fruits returned after deleting two");
+
+		//nothing happens when deleting tag that doesn't exist
+		$doc->removeTag("fruit","jellybean");
+
+		$fruit = $doc->getTags("fruit");
+		$this->assertEquals(count($fruit),1,"One fruits returned after attempting to delete non-existent fruit");
+
+		//delete the last fruit
+		$doc->removeTag("fruit","banana");
+
+		$fruit = $doc->getTags("fruit");
+		$this->assertNull($fruit,"All fruits deleted");
+
+		$tags = DataObject::get("DMSTag");
+		$this->assertEquals($tags->Count(),0,"No DMS tag objects remain after deletion");
+	}
+
 }
\ No newline at end of file