Sir Karl Popper's critical rationalism – a philosophy in the fallibilist tradition of Socrates, Kant and Peirce – is applied systematically to illuminate the values and principles underlying contemporary software development. The two aspects of Popper's philosophy, the natural and the social, provide a comprehensive and unified philosophical basis for understanding the newly emerged "agile" methodologies. It is argued in the first four sections of the paper – Philosophy of Science, Evolutionary Theory of Knowledge, class="EmphasisTypeItalic ">Metaphysics, and The Open Society – that the agile approach to software development is strongly endorsed by Popper's philosophy of critical rationalism. In the final section, the relevance of Christopher Alexander's ideas to agile methodologies and their similarity to Popper's philosophy is demonstrated.