Cellular biopolomers have the specific property of transforming chemical energy into mechanical work. This specificity is the basis of cellular movements in general, from cell division to cell motility. We describe here how actin assembly, in the absence or in the presence of molecular motors, can generate forces that eventually lead to cell movements. We present the state-of-the-art of single filament and many filament force generation. We show that in all cases, generic models can be built that are independent of the detailed molecular mechanism, in agreement with experimental results and quantifications based on stripped-down systems designed using purified proteins or cell extracts. Molecular features play a role only in the details of the mechanics.