The problem of the selection of forms and scales of convective flows in a horizontal layer of liquid heated from below is studied. The basic types of two- and three-dimensional flows, defects of roll structures, exchange of convective regimes, and different situations in which two-dimensional rolls have a preferred wave number are described. It is shown that when considered in terms of an optimal (preferred) wave number, to which a roll structure tends and which may or may not be reached depending on the overall geometry of the flow, the different experimental and theoretical results form a unified consistent picture. The basic methods used for investigation of convective structures are described.