The Pichia pastoris system has now been used successfully to express a large number of different intracellular and secreted proteins. In some cases, extremely high levels have been obtained (e.g., tumor necrosis factor, TNF [1 ]; human serum albumin, HSA [2 ]). The aim of this chapter is to illustrate some of the important practical considerations in obtaining efficient expression, to highlight the problems that can occur, and to describe the different approaches that have been used to solve them.