This chapter examines the third dimension of channel form: change in the elevation of the channel bed over distance along a river, or the longitudinal profile. Because the longitudinal profile of a river develops in conjunction with the development of river networks over geologic timescales, this topic is linked to the evolution of drainage basins. The main controls of profile form – water discharge, sediment discharge, and sediment caliber – are introduced. Both empirical and theoretical analyses of longitudinal profile development are presented. Downstream fining of bed material in rivers and the interplay of this phenomenon with longitudinal profile development is explored. At a more local scale, the factors that influence channel gradients are explored. Transient adjustments of the longitudinal profile in response to uplift or changes in base level are considered for bedrock and alluvial systems, including the development and migration of knickpoints. Variations in channel form in relation to the longitudinal profile are presented, with a detailed examination of step-pool channels in steep mountain streams.