Let G be a finite group. An action of G on an abelian group is called a G-module. If you reduce or complete at a prime p, the theory of G-modules breaks up into what Brauer called blocks, with no interaction between the different blocks. When p is large a block is just an irreducible representation of G. For primes dividing G a block usually contains many irreducible representations, and whatever you can build out of them. These days the subject of blocks is organized with homological algebra: to each block one attaches a triangulated category, or perhaps that category just is the block. I will explain some of this subject, and discuss how it changes – deforms is an appropriate word – when you replace abelian groups by KU-modules, i.e. by modules over the complex K-theory spectrum.