diff --git a/daggy/include/daggy/DAG.hpp b/daggy/include/daggy/DAG.hpp
index 75a9272..dfd6a60 100644
--- a/daggy/include/daggy/DAG.hpp
+++ b/daggy/include/daggy/DAG.hpp
@@ -1,9 +1,11 @@
 #pragma once
 
+#include <iostream>
 #include <deque>
 #include <stdexcept>
 #include <unordered_map>
 #include <unordered_set>
+#include <iterator>
 
 namespace daggy {
     template<typename T>
@@ -20,6 +22,15 @@ namespace daggy {
             // Returns the path from {from} to {to}
             std::deque<T> shortest_path(const T & from, const T & to);
 
+            // Breadth First Iterator
+            struct Iterator {
+              using iterator_category = std::random_access_iterator_tag;
+              using difference_type   = int;
+              using value_type        = T;
+              using point             = T*;
+              using reference         = T&;
+            };
+
         private:
             struct Vertex {
                 // It's a bit redundant to preserve both, but
@@ -34,5 +45,4 @@ namespace daggy {
     };
 
 #include "DAG.impl"
-
 }
diff --git a/daggy/include/daggy/DAG.impl b/daggy/include/daggy/DAG.impl
index f37d006..d3b96de 100644
--- a/daggy/include/daggy/DAG.impl
+++ b/daggy/include/daggy/DAG.impl
@@ -33,6 +33,10 @@ void DAG<T>::addEdge(const T & from, const T & to) {
   auto & src = vertices.at(from);
   auto & dst = vertices.at(to);
 
+  if (shortest_path(to, from).size() > 1) {
+    throw std::runtime_error("Unable to add edge that would result in a cycle");
+  }
+
   // Add the edge
   src.children.insert(to);
   dst.parents.insert(from);
diff --git a/tests/unit_dag.cpp b/tests/unit_dag.cpp
index 493790d..2879b16 100644
--- a/tests/unit_dag.cpp
+++ b/tests/unit_dag.cpp
@@ -13,12 +13,10 @@ TEST_CASE("DAG Basic Tests", "[dag]") {
       dag.addEdge(i-1, i);
     }
 
-    dag.addEdge(5, 9);
+    REQUIRE(dag.shortest_path(0,9).size() == 10);
 
-    auto sp = dag.shortest_path(1,9);
-    std::cout << "Shortest path from 1 to 10: (";
-    for (auto k : sp) {
-      std::cout << k << " -> ";
-    }
-    std::cout << std::endl;
+    dag.addEdge(5, 9);
+    REQUIRE(dag.shortest_path(0,9).size() == 7);
+
+    REQUIRE_THROWS(dag.addEdge(9, 5));
 }