Yeasts are widely used in production of recombinant proteins of medical or industrial interest. For each individual product, the most suitable expression system has to be identified and optimized, both on the genetic and fermentative level, by taking into account the properties of the product, the organism and the expression cassette. There is a wide range of important yeast expression hosts including the species Saccharomyces cerevisiae, Pichia pastoris, Hansenula polymorpha, Kluyveromyces lactis, Schizosaccharomyces pombe, Yarrowia lipolytica and Arxula adeninivorans, with various characteristics such as being thermo-tolerant or halo-tolerant, rapidly reaching high cell densities or utilizing unusual carbon sources. Several strains were also engineered to have further advantages, such as humanized glycosylation pathways or lack of proteases. Additionally, with a large variety of vectors. promoters and selection markers to choose from, combined with the accumulated knowledge on industrial-scale fermentation techniques and the current advances in the post-genomic technology, it is possible to design more cost-effective expression systems in order to meet the increasing demand for recombinant proteins and glycoproteins. In this review, the present status of the main and most promising yeast expression systems is discussed. (C) 2011 Elsevier Inc. All rights reserved.