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