Measurements of the internal properties of jets allow QCD to be studied at a new regime at a hadron collider. In this talk, we discuss recent measurements jet substructure and jet fragmentation that were performed using data collected by the ATLAS experiment at a centre-of-mass energy of √s=13 TeV. This includes a comprehensive suite of substructure observables measured for jets reconstructed with the soft-drop algorithm applied as well as a measurement of the Lund Plane using charged particles. The fragmentation properties of jets containing b-quarks will also be presented. All of the measurements are corrected for detector effects and are compared to the predictions of state-of-the-art Monte Carlo event generators.