Since 1985 the methylotrophic yeast Pichia pastoris has been successfully used for the production of over 300 recombinant proteins (1). (A brief description of published proteins with links to publications via PubMed is available at: http://public.kgi.edu/~cregg/Pichia.html). The increasing popularity of P. pastoris is attributed to: (a) the ease with which it can be genetically manipulated and cultured on a large scale; (b) its ability to produce foreign proteins either intracellularly or extracellularly at high levels; (c) its capability of performing many eukaryotic post-translational modifications, such as proteolytic processing, folding, disulfide bond formation, and glycosylation; and (d) its ready availability as a kit from Invitrogen Corporation (Carlsbad, CA). This article provides a general outline on the use of the P. pastoris expression system, summarizes the advantages and disadvantages of the system, and describes the characteristics of several new P. pastoris promoters, which may broaden the usefulness of the system. More detailed information on the P. pastoris expression system may be obtained in one of the many reviews on the system (2 and references therein) and from Invitrogen (http://www.invitrogen.com).