  The secondary, Roche-lobe filling stars in cataclysmic variables (CVs) are
key to our understanding of the origin, evolution and behaviour of this class
of interacting binary. We review the basic properties of the secondary stars in
CVs and the observational and analysis methods required to detect them. We then
describe the various astro-tomographic techniques which can be used to map the
surface intensity distribution of the secondary star, culminating in a detailed
explanation of Roche tomography. We conclude with a summary of the most
important results obtained to date and future prospects.
