Abstract. To any bounded analytic semigroup on Hilbert space or on L p -space, one may associate natural 'square functions'. In this survey paper, we review old and recent results on these square functions, as well as some extensions to various classes of Banach spaces, including noncommutative L p -spaces, Banach lattices, and their subspaces. We give some applications to H ∞ functional calculus, similarity problems, multiplier theory, and control theory. The purpose of this survey paper is to give a review of the various connections between functional calculus and square functions, as well as some of their applications to several aspects of the theory of semigroups. Of course it is only a selection of these applications, obviously influenced by the author's tastes.In Section 2, we briefly review all necessary definitions and basic results concerning sectorial operators, H ∞ functional calculus, and bounded analytic semigroups. We also include a subsection on Rademacher boundedness, a recent notion which now plays a key