As Christians, we affirm that Scripture is our supreme guide to truth and righteousness. Some wish to go further and assert that it is our only guide. But how then can we account for the remarkable insight and moral integrity that many unbelievers seem to display? Indeed, how to account for the myriad ways in which believers themselves navigate the world based on knowledge and intuition not always derived from Scripture? Enter the doctrine of natural law. Frequently misrepresented as an assertion of the autonomous power of human reason or as a uniquely Roman Catholic doctrine, natural law has actually been an integral part of orthodox Christian theology since the beginning, and is even clearly asserted in Scripture itself. In this brief guide, David Haines and Andrew Fulford explain the philosophical foundations of natural law, clear up common misunderstandings about the term, and demonstrate the robust biblical basis for natural law reasoning.