Werner Heisenberg was a pivotal figure in the development of quantum mechanics in the 1920s, and also one of its most insightful interpreters. Together with Bohr, Heisenberg forged what is commonly known as the 'Copenhagen interpretation'. Yet Heisenberg's philosophical viewpoint did not remain fixed over time, and his interpretation of quantum mechanics differed in several crucial respects from Bohr's. This book traces the development of Heisenberg's philosophy of quantum mechanics, beginning with his positivism of the mid-1920s, through his neo-Kantian reading of Bohr in the 1930s, and culminating with his 'linguistic turn' in the 1940s and 1950s. It focuses on the nature of this transformation in Heisenberg's thought and its wider philosophical context, which have up until now not received the attention they deserve. This new perspective on Heisenberg's interpretation of quantum mechanics will interest researchers and graduate students in the history and philosophy of twentieth-century physics.