Published online by Cambridge University Press: 12 April 2016
A brief review of the nature and subtypes of cataclysmic variables (CVs) is given. The catalogue of CVs is still very incomplete. All-sky surveys should add large numbers of CVs, which will improve knowledge of the space density of these systems. It is pointed out that the nova-like variables, which are the most difficult to discover, are the subtype having the highest space density. Their discovery is therefore the highest priority – they fix the frequency of CVs, which is important in population syntheses of binary stars.