Jets, i.e. collimated sprays of particles, are ubiquitous in particle phenomenology. I discuss QCD calculations which aim to describe the structure of jets. In particular, I focus on several techniques that have been developed in the past few years in order to better identify jets originating from the decay of boosted massive particles, showing how jet-mass distributions are modified when these grooming and tagging algorithms are applied to the jets and how a deeper understanding of QCD dynamics in this context can guide the development of better tools.