When trying to coax interesting (and possibly useful) quantum behaviour out of a system, we normally view dissipation as a nuisance whose effects should be minimized as much as possible. In this talk, I'll discuss a powerful and seemingly paradoxical approach where dissipation is deliberately harnessed to prepare interesting quantum states and functionalities. I'll focus on recent theory from my group showing how this strategy can be employed in quantum optomechanical systems, where mechanical motion interacts strongly with photons in a cavity via radiation pressure forces. Here, ‘engineered dissipation’ can allow the preparation quantum states of a mechanical resonator, entangled states of light, and even mediate non-reciprocal (i.e. uni-directional) interactions. These ideas have very recently been implemented experimentally to prepare non-classical states of picogram-scale mechanical resonators, where the relevant dissipation is produced by microwave photons in a superconducting quantum circuit.