Grain surface chemistry plays a crucial role in the formation of complex organic species in regions of star and planet formation. In my talk, I will speak about state-of-the-art models of chemistry on surfaces of interstellar grains. I will briefly discuss basic principles behind diffusive grain surface chemistry, recent relevant laboratory studies, and how to construct models that take them into account. I will show how models help us to understand molecular observations of star-forming regions, and present recent results.