The factors contributing to the structure (i.e. amplitude and shape) of echoes from major reflecting boundaries in tissues are reviewed. It is shown that the pulsed nature of a scanner's emitted sound field introduces complex variations in echo shape which are not usually taken into account. Moreover, echo structure is strongly modified by the geometry of the interface and not only by the physical composition of the adjacent materials. These effects are shown to be characteristic of the basic reflection process with pulsed fields. The influence of some receiver transfer properties on the displayed echo is also indicated. It is suggested that the analysis of sound fields scattered from small-scale internal structures (echo ensemble analysis) is a preferred method of tissue characterization.