diff --git a/gsl/multi_span b/gsl/multi_span
index 2186c7b..d91f02a 100644
--- a/gsl/multi_span
+++ b/gsl/multi_span
@@ -1451,7 +1451,7 @@ public:
     constexpr size_type length() const noexcept { return this->size(); }
 
     // length of the multi_span in bytes
-    constexpr size_type size_bytes() const noexcept { return sizeof(value_type) * this->size(); }
+    constexpr size_type size_bytes() const noexcept { return narrow_cast<size_type>(sizeof(value_type)) * this->size(); }
 
     // length of the multi_span in bytes
     constexpr size_type length_bytes() const noexcept { return this->size_bytes(); }
@@ -1653,7 +1653,8 @@ constexpr auto as_multi_span(multi_span<const byte, Dimensions...> s) noexcept -
              ConstByteSpan::bounds_type::static_size % narrow_cast<std::ptrdiff_t>(sizeof(U)) == 0),
         "Target type must be a trivial type and its size must match the byte array size");
 
-    Expects((s.size_bytes() % sizeof(U)) == 0 && (s.size_bytes() / sizeof(U)) < PTRDIFF_MAX);
+    Expects((s.size_bytes() % narrow_cast<std::ptrdiff_t>(sizeof(U))) == 0 && 
+            (s.size_bytes() / narrow_cast<std::ptrdiff_t>(sizeof(U))) < PTRDIFF_MAX);
     return {reinterpret_cast<const U*>(s.data()),
             s.size_bytes() / narrow_cast<std::ptrdiff_t>(sizeof(U))};
 }
diff --git a/gsl/span b/gsl/span
index 601580c..3a3fb52 100644
--- a/gsl/span
+++ b/gsl/span
@@ -494,7 +494,7 @@ public:
     constexpr index_type length() const noexcept { return size(); }
     constexpr index_type size() const noexcept { return storage_.size(); }
     constexpr index_type length_bytes() const noexcept { return size_bytes(); }
-    constexpr index_type size_bytes() const noexcept { return size() * sizeof(element_type); }
+    constexpr index_type size_bytes() const noexcept { return size() * narrow_cast<index_type>(sizeof(element_type)); }
     constexpr bool empty() const noexcept { return size() == 0; }
 
     // [span.elem], span element access
diff --git a/gsl/string_span b/gsl/string_span
index 703bc01..b5512a2 100644
--- a/gsl/string_span
+++ b/gsl/string_span
@@ -331,7 +331,7 @@ public:
     // Container signature should work for basic_string after C++17 version exists
     template <class Traits, class Allocator>
     constexpr basic_string_span(std::basic_string<element_type, Traits, Allocator>& str)
-        : span_(&str[0], str.length())
+        : span_(&str[0], narrow_cast<std::ptrdiff_t>(str.length()))
     {
     }
 
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 44db32d..c60f597 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -24,9 +24,9 @@ else()
     CHECK_CXX_COMPILER_FLAG("-std=c++14" COMPILER_SUPPORTS_CXX14)
     CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
     if(COMPILER_SUPPORTS_CXX14)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing -std=c++14 -O3 -Wall -Wno-missing-braces")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing -std=c++14 -O3 -Wall -Wextra -Wpedantic -Wno-missing-braces -Wconversion -Wsign-conversion -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Woverloaded-virtual")
     elseif(COMPILER_SUPPORTS_CXX11)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing -std=c++11 -O3 -Wall -Wno-missing-braces")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing -std=c++11 -O3 -Wall -Wextra -Wpedantic -Wno-missing-braces -Wconversion -Wsign-conversion -Wctor-dtor-privacy -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Woverloaded-virtual")
     else()
       message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
     endif()
diff --git a/tests/bounds_tests.cpp b/tests/bounds_tests.cpp
index d10bf6d..a4024d1 100644
--- a/tests/bounds_tests.cpp
+++ b/tests/bounds_tests.cpp
@@ -19,7 +19,7 @@
 #include <vector>
 
 using namespace std;
-using namespace gsl;;
+using namespace gsl;
 
 namespace 
 {
@@ -37,7 +37,7 @@ SUITE(bounds_test)
 			     j++)
 			{
 				use(j);
-				use(point[j]);
+				use(point[static_cast<size_t>(j)]);
 			}
 		}
 	}
diff --git a/tests/multi_span_tests.cpp b/tests/multi_span_tests.cpp
index c0240ea..8dbe151 100644
--- a/tests/multi_span_tests.cpp
+++ b/tests/multi_span_tests.cpp
@@ -1427,14 +1427,14 @@ SUITE(multi_span_tests)
             }
         }
 
-        size_t check_sum = 0;
+        auto check_sum = 0;
         for (auto i = 0; i < length; ++i) {
             check_sum += av[i][1];
         }
 
         {
             auto idx = 0;
-            size_t sum = 0;
+            auto sum = 0;
             for (auto num : section) {
                 CHECK(num == av[idx][1]);
                 sum += num;
@@ -1444,8 +1444,8 @@ SUITE(multi_span_tests)
             CHECK(sum == check_sum);
         }
         {
-            size_t idx = length - 1;
-            size_t sum = 0;
+            auto idx = length - 1;
+            auto sum = 0;
             for (auto iter = section.rbegin(); iter != section.rend(); ++iter) {
                 CHECK(*iter == av[idx][1]);
                 sum += *iter;
@@ -1650,7 +1650,7 @@ SUITE(multi_span_tests)
         {
             multi_span<int, dynamic_range> av = a;
             auto wav = as_writeable_bytes(av);
-            CHECK(wav.data() == (byte*) &a[0]);
+            CHECK(wav.data() == reinterpret_cast<byte*>(&a[0]));
             CHECK(wav.length() == sizeof(a));
         }
     }
diff --git a/tests/notnull_tests.cpp b/tests/notnull_tests.cpp
index 526b074..201b797 100644
--- a/tests/notnull_tests.cpp
+++ b/tests/notnull_tests.cpp
@@ -83,7 +83,7 @@ SUITE(NotNullTests)
         not_null<Unrelated*> s = reinterpret_cast<Unrelated*>(p);
 #endif
         not_null<Unrelated*> t = reinterpret_cast<Unrelated*>(p.get());
-        CHECK((void*)p.get() == (void*)t.get());
+        CHECK(reinterpret_cast<void*>(p.get()) == reinterpret_cast<void*>(t.get()));
     }
 
     TEST(TestNotNullAssignment)
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index 35e6b03..9ef2721 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -1336,8 +1336,8 @@ SUITE(span_tests)
 #endif
         {
             auto f = [&]() {
-                span<int, 4> s4 = {arr2, 2};
-                static_cast<void>(s4);
+                span<int, 4> _s4 = {arr2, 2};
+                static_cast<void>(_s4);
             };                     
             CHECK_THROW(f(), fail_fast);
         }
@@ -1345,8 +1345,8 @@ SUITE(span_tests)
         // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
         span<int> av = arr2;
         auto f = [&]() {
-            span<int, 4> s4 = av;
-            static_cast<void>(s4);            
+            span<int, 4> _s4 = av;
+            static_cast<void>(_s4);            
         };
         CHECK_THROW(f(), fail_fast);
     }
diff --git a/tests/strided_span_tests.cpp b/tests/strided_span_tests.cpp
index 86666d1..de10910 100644
--- a/tests/strided_span_tests.cpp
+++ b/tests/strided_span_tests.cpp
@@ -701,7 +701,7 @@ SUITE(strided_span_tests)
 
         int s = sizeof(int) / sizeof(byte);
         auto d2 = 3 * s;
-        auto d1 = sizeof(int) * 12 / d2;
+        auto d1 = narrow_cast<int>(sizeof(int)) * 12 / d2;
 
         // convert to 4x12 array of bytes
         auto av = as_multi_span(as_bytes(as_multi_span(arr, 4)), dim(d1), dim(d2));
diff --git a/tests/utils_tests.cpp b/tests/utils_tests.cpp
index 9f4ba02..781f692 100644
--- a/tests/utils_tests.cpp
+++ b/tests/utils_tests.cpp
@@ -67,7 +67,7 @@ SUITE(utils_tests)
     }
 
     int j = 0;
-    void g() { j += 1; };
+    void g() { j += 1; }
     TEST(finally_function_ptr)
     {
         j = 0;