The peculiar A stars were first noted as peculiar because they exhibit anomalously strong lines of one or more of the elements Si, Cr, Mn, Sr, and Eu. These diverse objects are all called Ap stars because their assigned spectral types fall predominantly in the range B9-F0. Recent studies have suggested, however, that the Ap stars should be subdivided into two groups. The stars characterized by enhanced lines of Mn, and usually Hg as well, differ from the remaining Ap stars in composition and binary frequency; there are no confirmed spectrum, photometric, or magnetic variations in any Hg-Mn stars; and the upper limit on the magnetic field strengths of the Hg-Mn stars is about 200 gauss. Because of these differences, many observers (e.g. Sargent and Searle 1967; Preston 1971b; Wolff and Wolff 1974) have suggested that the Hg-Mn stars are fundamentally different in their properties, and possibly in their origin and evolution, from the SiCrEuSr stars. We shall be concerned today only with this latter group of stars, and in the review that follows only members of the SiCrEuSr class of stars will be referred to as Ap stars.