In this thesis we study aspects of jet substructure through first principles calculations in Quantum Chromo Dynamics (QCD). To begin with, a short introduction to QCD, paying particular attention to the soft and collinear limits, is presented, followed by a brief review of jet substructure. Following this review material, the next-to-next-to-leading logarithmic (NNLL) structure of the groomed jet mass is investigated at second order in the strong cooupling. This is done by carrying out a next-to-leading-order calculation in the collinear limit of the heavy-hemisphere mass distribution, where each hemisphere has been groomed with the modified mass drop tagger (mMDT), starting from the the triple collinear splitting functions. This calculation sheds light to the relationship between the triple collinear splitting function and NNLL structure of collinear logarithms, which is important for efforts to include triple collinear splitting functions in parton showers . It also provides the insight needed to carry out the NNLL resummation for distributions of groomed observables, which is the subject of chapter 5. This has potential applications in precision studies of groomed observables at the LHC, or other collider. In chapter 6, we investigate a method for tagging boosted top quarks, using a combination of grooming, prong finding, and cutting on a jet shape, by carrying out resummed calculations for the tagged fraction of both signal and background jets. This allows us to understand, from first principles, the the interplay between the different steps involved in the tagging procedure.