Chapter 4 covers the evolution of the solid component of protoplanetary disks, from dust through to the formation of km-scale planetesimals. The physics of how dust particles interact with the gas through aerodynamic drag is described, together with the consequences - vertical settling, radial drift, particle trapping, and particle pile-up. The outcome of particle collisions, and their theoretical description using the coagulation equation, are reviewed. Collective mechanisms for planetesimal formation via gravitational collapse are discussed, starting with the classical Goldreich-Ward theory, and concluding with the streaming instability of aerodynamically coupled mixtures of gas and dust.