First the concepts of time and of time scales are discussed. Then the paper deals with the definition of time scales and with the relevant resolutions and recommendations of international organizations. After a discussion of frequency and time standards, at present of importance with respect to time scale production, clock models describing the clock behavior are treated. This is followed by a review of problems of time comparisons and a description of time service equipment and methods. Then averaging and extrapolation problems are discussed. The present qualities of the international atomic time and of atomic time scales produced by time-keeping institutes are finally reviewed.