The study of collective nonlinear dynamics of coupled mechanical resonators is regaining attention in recent years thanks to rapid developments in the fields of microelectromechanical and nanoelectromechanical systems (MEMS and NEMS). We review a wide range of collective dynamical phenomena, while highlighting the common concepts and theoretical tools that we have developed for treating them. We provide detailed derivations of amplitude equations, which allow us to obtain reduced descriptions for the relevant dynamics of our complex systems. We apply these amplitude equations to study (a) resonant response to parametric excitation; (b) pattern selection, or the nonlinear competition between extended modes in situations of multistability; (c) formation and dynamics of intrinsically localized modes (ILM); and (d) spontaneous synchronization of oscillators with differing frequencies. All the predictions obtained from analyzing the different amplitude equations are in excellent agreement with numerical solutions of the underlying equations of motion, suggesting that the predicted effects can be observed in arrays of real micromechanical or nanomechanical resonators, thus motivating new experiments in these systems.