Statistical mechanics is our tool for deriving the laws that emerge from complex systems. Sethna's text distills the subject to be accessible to those in all realms of science and engineering - avoiding extensive use of quantum mechanics, thermodynamics, and molecular physics. This book explains how bacteria search for food, and how DNA replication is proof-read in biology; optimises data compression, and explains transitions in complexity in computer science; explains the onset of chaos, and launched random matrix theory in mathematics; addresses extreme events in engineering; and models pandemics and language usage in the social sciences. Sethna's exercises introduce physicists to these triumphs and a hundred others - broadening the horizons of scholars both practicing and nascent.