diff --git a/library/Businessprocess/Exception/ModificationError.php b/library/Businessprocess/Exception/ModificationError.php
new file mode 100644
index 0000000..430d513
--- /dev/null
+++ b/library/Businessprocess/Exception/ModificationError.php
@@ -0,0 +1,9 @@
+<?php
+
+namespace Icinga\Module\Businessprocess\Exception;
+
+use Icinga\Exception\IcingaException;
+
+class ModificationError extends IcingaException
+{
+}
diff --git a/library/Businessprocess/Modification/NodeAction.php b/library/Businessprocess/Modification/NodeAction.php
index c93f13a..e18001b 100644
--- a/library/Businessprocess/Modification/NodeAction.php
+++ b/library/Businessprocess/Modification/NodeAction.php
@@ -3,6 +3,7 @@
 namespace Icinga\Module\Businessprocess\Modification;
 
 use Icinga\Module\Businessprocess\BpConfig;
+use Icinga\Module\Businessprocess\Exception\ModificationError;
 use Icinga\Module\Businessprocess\Node;
 use Icinga\Exception\ProgrammingError;
 
@@ -48,10 +49,13 @@ abstract class NodeAction
     abstract public function applyTo(BpConfig $config);
 
     /**
-     * Every NodeAction must be able to tell whether it could be applied to a BusinessProcess
+     * Every NodeAction must be able to tell whether it can be applied to a BusinessProcess
      *
-     * @param BpConfig $config
-     * @return bool
+     * @param   BpConfig    $config
+     *
+     * @throws  ModificationError
+     *
+     * @return  bool
      */
     abstract public function appliesTo(BpConfig $config);
 
@@ -81,6 +85,21 @@ abstract class NodeAction
         return $this->getActionName() === $actionName;
     }
 
+    /**
+     * Throw a ModificationError
+     *
+     * @param   string  $msg
+     * @param   mixed   ...
+     *
+     * @throws  ModificationError
+     */
+    protected function error($msg)
+    {
+        $error = ModificationError::create(func_get_args());
+        /** @var ModificationError $error */
+        throw $error;
+    }
+
     /**
      * Create an instance of a given actionName for a specific Node
      *
diff --git a/library/Businessprocess/Modification/NodeAddChildrenAction.php b/library/Businessprocess/Modification/NodeAddChildrenAction.php
index 65e8444..def37a1 100644
--- a/library/Businessprocess/Modification/NodeAddChildrenAction.php
+++ b/library/Businessprocess/Modification/NodeAddChildrenAction.php
@@ -17,11 +17,11 @@ class NodeAddChildrenAction extends NodeAction
     {
         $name = $this->getNodeName();
 
-        if (! $config->hasNode($name)) {
-            return false;
+        if (! $config->hasBpNode($name)) {
+            $this->error('Process "%s" not found', $name);
         }
 
-        return $config->getNode($name) instanceof BpNode;
+        return true;
     }
 
     /**
diff --git a/library/Businessprocess/Modification/NodeCreateAction.php b/library/Businessprocess/Modification/NodeCreateAction.php
index 69dc77c..e33c128 100644
--- a/library/Businessprocess/Modification/NodeCreateAction.php
+++ b/library/Businessprocess/Modification/NodeCreateAction.php
@@ -72,7 +72,17 @@ class NodeCreateAction extends NodeAction
      */
     public function appliesTo(BpConfig $config)
     {
-        return ! $config->hasNode($this->getNodeName());
+        $name = $this->getNodeName();
+        if ($config->hasNode($name)) {
+            $this->error('A node with name "%s" already exists', $name);
+        }
+
+        $parent = $this->getParentName();
+        if ($parent !== null && !$config->hasBpNode($parent)) {
+            $this->error('Parent process "%s" missing', $parent);
+        }
+
+        return true;
     }
 
     /**
diff --git a/library/Businessprocess/Modification/NodeModifyAction.php b/library/Businessprocess/Modification/NodeModifyAction.php
index 8cb240e..1b33094 100644
--- a/library/Businessprocess/Modification/NodeModifyAction.php
+++ b/library/Businessprocess/Modification/NodeModifyAction.php
@@ -47,15 +47,21 @@ class NodeModifyAction extends NodeAction
         $name = $this->getNodeName();
 
         if (! $config->hasNode($name)) {
-            return false;
+            $this->error('Node "%s" not found', $name);
         }
 
         $node = $config->getNode($name);
 
         foreach ($this->properties as $key => $val) {
-            $func = 'get' . ucfirst($key);
-            if ($this->formerProperties[$key] !== $node->$func()) {
-                return false;
+            $currentVal = $node->{'get' . ucfirst($key)}();
+            if ($this->formerProperties[$key] !== $currentVal) {
+                $this->error(
+                    'Property %s of node "%s" changed its value from "%s" to "%s"',
+                    $key,
+                    $name,
+                    $this->formerProperties[$key],
+                    $currentVal
+                );
             }
         }
 
diff --git a/library/Businessprocess/Modification/NodeMoveAction.php b/library/Businessprocess/Modification/NodeMoveAction.php
index f7428fc..c25bafb 100644
--- a/library/Businessprocess/Modification/NodeMoveAction.php
+++ b/library/Businessprocess/Modification/NodeMoveAction.php
@@ -72,46 +72,55 @@ class NodeMoveAction extends NodeAction
     public function appliesTo(BpConfig $config)
     {
         if (! $config->getMetadata()->isManuallyOrdered()) {
-            return false;
+            $this->error('Process configuration is not manually ordered yet');
         }
 
         $name = $this->getNodeName();
         if ($this->parent !== null) {
             if (! $config->hasBpNode($this->parent)) {
-                return false;
+                $this->error('Parent process "%s" missing', $this->parent);
             }
             $parent = $config->getBpNode($this->parent);
             if (! $parent->hasChild($name)) {
-                return false;
+                $this->error('Node "%s" not found in process "%s"', $name, $this->parent);
             }
 
             $nodes = $parent->getChildNames();
             if (! isset($nodes[$this->from]) || $nodes[$this->from] !== $name) {
-                return false;
+                $this->error('Node "%s" not found at position %d', $name, $this->from);
             }
         } else {
             if (! $config->hasNode($name)) {
-                return false;
+                $this->error('Toplevel process "%s" not found', $name);
             }
 
-            if ($config->getBpNode($name)->getDisplay() !== $this->getFrom()) {
-                return false;
+            if ($config->getBpNode($name)->getDisplay() !== $this->from) {
+                $this->error('Toplevel process "%s" not found at position %d', $name, $this->from);
             }
         }
 
         if ($this->parent !== $this->newParent) {
             if ($this->newParent !== null) {
                 if (! $config->hasBpNode($this->newParent)) {
-                    return false;
+                    $this->error('New parent process "%s" missing', $this->newParent);
                 }
 
                 $childrenCount = $config->getBpNode($this->newParent)->countChildren();
+                if ($this->to > 0 && $childrenCount < $this->to) {
+                    $this->error(
+                        'New parent process "%s" has not enough children. Target position %d out of range',
+                        $this->newParent,
+                        $this->to
+                    );
+                }
             } else {
                 $childrenCount = $config->countChildren();
-            }
-
-            if ($this->getTo() > 0 && $childrenCount < $this->getTo()) {
-                return false;
+                if ($this->to > 0 && $childrenCount < $this->to) {
+                    $this->error(
+                        'Process configuration has not enough toplevel processes. Target position %d out of range',
+                        $this->to
+                    );
+                }
             }
         }
 
diff --git a/library/Businessprocess/Modification/NodeRemoveAction.php b/library/Businessprocess/Modification/NodeRemoveAction.php
index 09a20d2..fd8aa2b 100644
--- a/library/Businessprocess/Modification/NodeRemoveAction.php
+++ b/library/Businessprocess/Modification/NodeRemoveAction.php
@@ -40,12 +40,21 @@ class NodeRemoveAction extends NodeAction
      */
     public function appliesTo(BpConfig $config)
     {
+        $name = $this->getNodeName();
         $parent = $this->getParentName();
         if ($parent === null) {
-            return $config->hasNode($this->getNodeName());
+            if (!$config->hasNode($name)) {
+                $this->error('Toplevel process "%s" not found', $name);
+            }
         } else {
-            return $config->hasNode($this->getNodeName()) && $config->hasNode($this->getParentName());
+            if (! $config->hasNode($parent)) {
+                $this->error('Parent process "%s" missing', $parent);
+            } elseif (! $config->getBpNode($parent)->hasChild($name)) {
+                $this->error('Node "%s" not found in process "%s"', $name, $parent);
+            }
         }
+
+        return true;
     }
 
     /**