Abstract: Living cells exert force in many basic processes. Forces cleave the mother cell into two daughters during cytokinesis in cell division, enable rigidity sensing in cell growth and differentiation, apply traction during cell migration and stimulate extracellular matrix reorganization during wound healing. For these and many other purposes cells assemble and operate complex multiprotein cytoskeletal machines. I will discuss assembly and function of contractile cellular machines built from actin filaments, force-producing myosin motor proteins and other components. (1) How are these machines assembled? We studied the contractile cytokinetic ring in fission yeast using time-resolved quantitative confocal microscopy and computer simulations. Assembly occurs via a remarkably stochastic "search, capture, pull and release" mechanism whereby ~63 membrane-bound precursor nodes are condensed into a continuous ring when node-bound myosins pull on transient formin-nucleated node-node actin connectors. (2) How do the machines work? We discuss stress fibers, possibly the most accessible cellular contractile force-producing machines. We quantitatively modeled their kinetics as dramatically revealed by two recent experimental studies.