diff --git a/include/gsl.h b/include/gsl.h
index 4eb555b..f4c9178 100644
--- a/include/gsl.h
+++ b/include/gsl.h
@@ -19,10 +19,10 @@
 #ifndef GSL_GSL_H
 #define GSL_GSL_H
 
-#include "gsl_assert.h"  // Ensures/Expects
-#include "gsl_util.h"    // finally()/narrow()/narrow_cast()...
+#include "gsl_assert.h"     // Ensures/Expects
+#include "gsl_util.h"       // finally()/narrow()/narrow_cast()...
 #include "span.h"           // span
-#include "multi_span.h"           // multi_span, strided_span...
+#include "multi_span.h"     // multi_span, strided_span...
 #include "string_span.h"    // zstring, string_span, zstring_builder...
 #include <memory>
 
diff --git a/include/span.h b/include/span.h
index 053b488..3f3b8fe 100644
--- a/include/span.h
+++ b/include/span.h
@@ -192,7 +192,7 @@ public:
 
     constexpr const_span_iterator& operator--() noexcept
     {
-        Expects(span_ && index > 0 && index_ <= span_->length());
+        Expects(span_ && index_ > 0 && index_ <= span_->length());
         --index_;
         return *this;
     }
@@ -366,7 +366,7 @@ public:
     using reference = element_type&;
 
     using iterator = details::span_iterator<span<ElementType, Extent>>;
-    using const_iterator = details::span_iterator<span>;
+    using const_iterator = details::const_span_iterator<span>;
     using reverse_iterator = std::reverse_iterator<iterator>;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
@@ -512,13 +512,17 @@ public:
     // [span.iter], span iterator support 
     iterator begin() const noexcept { return {this, 0}; }
     iterator end() const noexcept { return {this, length()}; }
+
+    const_iterator cbegin() const noexcept { return {this, 0}; }
+    const_iterator cend() const noexcept { return {this, length()}; }
  
-    reverse_iterator rbegin() const noexcept { return {this, length()}; }
-    reverse_iterator rend() const noexcept { return {this, 0}; }
+    reverse_iterator rbegin() const noexcept { return reverse_iterator{{this, length()}}; }
+    reverse_iterator rend() const noexcept { return reverse_iterator{{this, 0}}; }
+
+    const_reverse_iterator crbegin() const noexcept { return reverse_iterator{{this, length()}}; }
+    const_reverse_iterator crend() const noexcept { return reverse_iterator{{this, 0}}; }
 
 private:
-    constexpr static const bool is_span_type = true;
-
     template <index_type Extent>
     class extent_type;
     
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index ae3c32e..7652ff9 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -799,8 +799,6 @@ SUITE(span_tests)
         span<int>::const_iterator it1;
         span<int>::const_iterator it2;
         CHECK(it1 == it2);
-
-        
     }
 
     TEST(begin_end)
@@ -809,12 +807,12 @@ SUITE(span_tests)
             int a[] = { 1, 2, 3, 4 };
             span<int> s = a;
 
-            span<int>::iterator it = s.begin();
+            auto it = s.begin();
             auto first = it;
             CHECK(it == first);
             CHECK(*it == 1);
 
-            span<int>::iterator beyond = s.end();
+            auto beyond = s.end();
             CHECK(it != beyond);
             CHECK_THROW(*beyond, fail_fast);
 
@@ -845,6 +843,132 @@ SUITE(span_tests)
         }
     }
 
+    TEST(cbegin_cend)
+    {
+        {
+            int a[] = {1, 2, 3, 4};
+            span<int> s = a;
+
+            auto it = s.cbegin();
+            auto first = it;
+            CHECK(it == first);
+            CHECK(*it == 1);
+
+            auto beyond = s.cend();
+            CHECK(it != beyond);
+            CHECK_THROW(*beyond, fail_fast);
+
+            CHECK(beyond - first == 4);
+            CHECK(first - first == 0);
+            CHECK(beyond - beyond == 0);
+
+            ++it;
+            CHECK(it - first == 1);
+            CHECK(*it == 2);
+            *it = 22;
+            CHECK(*it == 22);
+            CHECK(beyond - it == 3);
+
+            it = first;
+            CHECK(it == first);
+            while (it != s.cend())
+            {
+                *it = 5;
+                ++it;
+            }
+
+            CHECK(it == beyond);
+            CHECK(it - beyond == 0);
+
+            for (auto& n : s)
+                CHECK(n == 5);
+        }
+    }
+
+    TEST(rbegin_rend)
+    {
+        {
+            int a[] = {1, 2, 3, 4};
+            span<int> s = a;
+
+            auto it = s.rbegin();
+            auto first = it;
+            CHECK(it == first);
+            CHECK(*it == 4);
+
+            auto beyond = s.rend();
+            CHECK(it != beyond);
+            CHECK_THROW(*beyond, fail_fast);
+
+            CHECK(beyond - first == 4);
+            CHECK(first - first == 0);
+            CHECK(beyond - beyond == 0);
+
+            ++it;
+            CHECK(it - first == 1);
+            CHECK(*it == 3);
+            *it = 22;
+            CHECK(*it == 22);
+            CHECK(beyond - it == 3);
+
+            it = first;
+            CHECK(it == first);
+            while (it != s.rend())
+            {
+                *it = 5;
+                ++it;
+            }
+
+            CHECK(it == beyond);
+            CHECK(it - beyond == 0);
+
+            for (auto& n : s)
+                CHECK(n == 5);
+        }
+    }
+
+    TEST(crbegin_crend)
+    {
+        {
+            int a[] = {1, 2, 3, 4};
+            span<int> s = a;
+
+            auto it = s.crbegin();
+            auto first = it;
+            CHECK(it == first);
+            CHECK(*it == 4);
+
+            auto beyond = s.crend();
+            CHECK(it != beyond);
+            CHECK_THROW(*beyond, fail_fast);
+
+            CHECK(beyond - first == 4);
+            CHECK(first - first == 0);
+            CHECK(beyond - beyond == 0);
+
+            ++it;
+            CHECK(it - first == 1);
+            CHECK(*it == 3);
+            *it = 22;
+            CHECK(*it == 22);
+            CHECK(beyond - it == 3);
+
+            it = first;
+            CHECK(it == first);
+            while (it != s.crend())
+            {
+                *it = 5;
+                ++it;
+            }
+
+            CHECK(it == beyond);
+            CHECK(it - beyond == 0);
+
+            for (auto& n : s)
+                CHECK(n == 5);
+        }
+    }
+
     TEST(comparison_operators)
     {
         {