diff --git a/dag/dag.go b/dag/dag.go
new file mode 100644
index 000000000..e41de6053
--- /dev/null
+++ b/dag/dag.go
@@ -0,0 +1,14 @@
+package dag
+
+// AcyclicGraph is a specialization of Graph that cannot have cycles. With
+// this property, we get the property of sane graph traversal.
+type AcyclicGraph struct {
+	*Graph
+}
+
+// WalkFunc is the callback used for walking the graph.
+type WalkFunc func(Vertex)
+
+// Walk walks the graph, calling your callback as each node is visited.
+func (g *AcyclicGraph) Walk(cb WalkFunc) {
+}
diff --git a/dag/graph.go b/dag/graph.go
index b1cbd84d4..4d4450cc4 100644
--- a/dag/graph.go
+++ b/dag/graph.go
@@ -26,9 +26,6 @@ type NamedVertex interface {
 	Name() string
 }
 
-// WalkFunc is the callback used for walking the graph.
-type WalkFunc func(Vertex)
-
 // Vertices returns the list of all the vertices in the graph.
 func (g *Graph) Vertices() []Vertex {
 	return g.vertices
@@ -81,10 +78,6 @@ func (g *Graph) Connect(edge Edge) {
 	s.Add(source)
 }
 
-// Walk walks the graph, calling your callback as each node is visited.
-func (g *Graph) Walk(cb WalkFunc) {
-}
-
 // String outputs some human-friendly output for the graph structure.
 func (g *Graph) String() string {
 	var buf bytes.Buffer
diff --git a/terraform/graph_config.go b/terraform/graph_config.go
index ca02c697d..716241ca7 100644
--- a/terraform/graph_config.go
+++ b/terraform/graph_config.go
@@ -39,8 +39,12 @@ func Graph2(mod *module.Tree) (*dag.Graph, error) {
 	}
 
 	// Write all the modules out
+	children := mod.Children()
 	for _, m := range config.Modules {
-		nodes = append(nodes, &GraphNodeConfigModule{Module: m})
+		nodes = append(nodes, &GraphNodeConfigModule{
+			Module: m,
+			Tree:   children[m.Name],
+		})
 	}
 
 	// Build the full map of the var names to the nodes.
diff --git a/terraform/graph_config_node.go b/terraform/graph_config_node.go
index cc0e43202..3fa479b8a 100644
--- a/terraform/graph_config_node.go
+++ b/terraform/graph_config_node.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 
 	"github.com/hashicorp/terraform/config"
+	"github.com/hashicorp/terraform/config/module"
 	"github.com/hashicorp/terraform/dag"
 )
 
@@ -27,6 +28,7 @@ type graphNodeConfig interface {
 // GraphNodeConfigModule represents a module within the configuration graph.
 type GraphNodeConfigModule struct {
 	Module *config.Module
+	Tree   *module.Tree
 }
 
 func (n *GraphNodeConfigModule) Name() string {
diff --git a/terraform/semantics.go b/terraform/semantics.go
index 51c17b68a..566a3c279 100644
--- a/terraform/semantics.go
+++ b/terraform/semantics.go
@@ -3,9 +3,70 @@ package terraform
 import (
 	"fmt"
 
+	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/terraform/config"
+	"github.com/hashicorp/terraform/dag"
 )
 
+// GraphSemanticChecker is the interface that semantic checks across
+// the entire Terraform graph implement.
+//
+// The graph should NOT be modified by the semantic checker.
+type GraphSemanticChecker interface {
+	Check(*dag.Graph) error
+}
+
+// UnorderedSemanticCheckRunner is an implementation of GraphSemanticChecker
+// that runs a list of SemanticCheckers against the vertices of the graph
+// in no specified order.
+type UnorderedSemanticCheckRunner struct {
+	Checks []SemanticChecker
+}
+
+func (sc *UnorderedSemanticCheckRunner) Check(g *dag.Graph) error {
+	var err error
+	for _, v := range g.Vertices() {
+		for _, check := range sc.Checks {
+			if e := check.Check(g, v); e != nil {
+				err = multierror.Append(err, e)
+			}
+		}
+	}
+
+	return err
+}
+
+// SemanticChecker is the interface that semantic checks across the
+// Terraform graph implement. Errors are accumulated. Even after an error
+// is returned, child vertices in the graph will still be visited.
+//
+// The graph should NOT be modified by the semantic checker.
+//
+// The order in which vertices are visited is left unspecified, so the
+// semantic checks should not rely on that.
+type SemanticChecker interface {
+	Check(*dag.Graph, dag.Vertex) error
+}
+
+// SemanticCheckModulesExist is an implementation of SemanticChecker that
+// verifies that all the modules that are referenced in the graph exist.
+type SemanticCheckModulesExist struct{}
+
+// TODO: test
+func (*SemanticCheckModulesExist) Check(g *dag.Graph, v dag.Vertex) error {
+	mn, ok := v.(*GraphNodeConfigModule)
+	if !ok {
+		return nil
+	}
+
+	if mn.Tree == nil {
+		return fmt.Errorf(
+			"module '%s' not found", mn.Module.Name)
+	}
+
+	return nil
+}
+
 // smcUserVariables does all the semantic checks to verify that the
 // variables given satisfy the configuration itself.
 func smcUserVariables(c *config.Config, vs map[string]string) []error {