This chapter offers a detailed understanding of the mechanism of sound production in brass instruments, drawing on recent scientific research but presented in language easily comprehended by readers unfamiliar with scientific writing. Sections of the chapter explain the brass instrument player’s actions in blowing, and how the lips vibrate. The resonances of the air inside the tubing of a brass instrument are discussed, and their important influence on the vibration of the lips is described. The concept of ‘slotting’, familiar to brass players, is explained. Further sections deal with the contributions of the mouthpiece and bell flare to a well-designed brass instrument. The concept of a ‘cooperative regime’, less familiar to musicians but accounting for important aspects of the playing experience, is explained. Finally, the sound quality of brass instruments is discussed, in particular how it changes between soft and loud playing and how this effect depends on instrument design, introducing the phenomenon of ‘spectral enrichment’.