This chapter focuses on mass transfer. Mass transfer can occur within bodies via Fick’s law between a material and a fluid medium through convection. Fick's law is introduced, along with the variables involved in calculating mass diffusion. Diffusion in gases, liquids, non-porous materials, and porous materials are analyzed, with guidance on obtaining the respective diffusion coefficients. The differential equation governing mass diffusion is derived, along with the corresponding boundary conditions. Both steady-state and transient analyses are carried out. The next topic addressed in the chapter is convective mass transfer, introducing the theory of concentration boundary layers and flow regimes. Dimensionless mass transfer numbers are presented, as well as the Chilton-Colburn analogy between heat and mass transfer. Working equations for convective mass transfer are provided, with examples of application to typical mass transfer conditions.