We consider suspensions of many small rigid particles in a fluid. The theoretical study of the effective rheology, i.e. fluid properties, of such complex flows dates back to Einstein's doctoral research, where he predicted an effective increase of the fluid viscosity due to the presence of the particle. Despite its relevance for many applications and a vast physics and engineering literature on the topic, a mathematical rigorous derivation of effective rheology has been elusive until recent years. In this talk, I give an overview of results that not only justify Einstein's formal calculations but go far beyond. In particular, fascinating non-Newtonian effects will be presented.