The principal chemical mechanisms relevant to the growth of diamond from gaseous hydrogen and hydrocarbon species are presented. The kinetic processes occurring during the activation and transport of the gaseous species to the growing surface are described, with the key processes being the generation and subsequent reactions of gaseous atomic hydrogen. The structure, composition, and dynamics of the growing surface are discussed. A simple, non-stereospecific model of the surface growth process is presented which reveals most of the general characteristics of the growth process, such as the H atom flux dependence of growth rate and quality. A detailed model of growth at the (110) surface sites from single carbon reactants then follows, which highlights the key role of gaseous atomic hydrogen abstractions of hydrogen from the surface. The extension of this understanding to chemistries containing oxygen and halogen species is indicated.