Jet Substructure at the LHC

Jets are collimated sprays of energetic hadrons that are copiously produced at the LHC. The substructure of jets is an important tool in the search for new physics at the LHC. I will mostly focus on two (quite different) examples: the mass and electric charge of a jet. Their calculation from first principles in QCD will be presented and compared to predictions from Monte Carlo programs. I will conclude by discussing some recent work on calculations for more differential measurements.