Plastic deformation of particulate aggregates at elevated temperatures constitutes a critical step in the fabrication of high temperature materials by the powder processing route. Substantial improvements observed in the microstructures and the mechanical properties of the hot-worked products can be attributed not only to the continued densification through a reduction in porosity, but also to the progressive strengthening of the matrix during hot working. This paper describes the salient theoretical aspects of the plastic deformation of compressible solids with special emphasis on the applicable flow models. The concepts of yield criteria and the flow rule for porous bodies are introduced and the effects of geometric hardening and strain hardening of the matrix on the macroscopic flow behavior are described in detail. Experimental studies on the deformation behavior of aluminum alloy (X7091) powder compacts at 400°C have clearly established the validity of the flow model in terms the relationships between flow stress, true strain, and the relative density. The paper also discusses the successful application of the plasticity theory developed in this work to the problem of hot extrusion of powder consolidated billets.