This open access book introduces graduate students with some collider phenomenology expertise to the rapidly advancing field of jet substructure physics. The second edition improves the presentation of theoretical topics, including soft factorization, and offers updates on novel jet substructure algorithms, tools, and recent experimental findings. Additionally, it introduces the Lund jet plane, a new substructure tool with diverse applications, in a dedicated chapter covering its theoretical properties, use in jet tagging, and related experimental results. The notes start with a phenomenological rationale for the importance of studying jets and their substructure at the LHC. They provide concise introductions to QCD and hadron-collider phenomenology. The concept of jets, constructed using sequential recombination algorithms, is introduced, along with relevant experimental aspects. The notes then delve into the multi-scale nature of jet substructure calculations and discuss the foundations of these calculations for basic jet quantities.Readers then explore jet substructure concepts, tools, and analytical calculations. The book concludes with a review of LHC searches, summarizing insights and outlining future directions
