'A beautiful exposition of the holomorphic side of martingale theory, where Hardy martingales play the leading role, with many deep applications to Banach spaces. Unlike most books on martingale theory where convexity is central, Müller's remarkable and unique book places the emphasis on the martingales that arise from averaging the boundary values of analytic functions in Hardy spaces. The latter discretize the continuous martingales obtained by composing an analytic function with complex Brownian motion. Consideration of the Banach space valued case leads to deep geometric applications.' Gilles Pisier, Texas A&M