John Stevens Henslow is known for his formative influence on Charles Darwin, who described their meeting as the one circumstance 'which influenced my career more than any other'. As Professor of Botany at Cambridge University, Henslow was Darwin's teacher and eventual lifelong friend, but what of the man himself? In this biography, much previously unpublished material has been carefully sifted and selected to produce a rounded picture of a remarkable and unusually likeable academic. The time in 1829-31 when Darwin 'walked with Henslow' in and around Cambridge was followed directly by Darwin's voyage around the world. The gradually changing relationship between teacher and pupil over the course of time is revealed through their correspondence, illuminating a remarkable friendship which persisted, in spite of Darwin's eventual atheism and Henslow's never-failing liberal Christian belief, to the end of Henslow's life.