Genetic and biochemical techniques are being employed to study the transport and compartmentalization of proteins in Saccharomyces cerevisiae. A number of thermosensitive sec mutants have been isolated which are blocked at different stages of the secretory route (endoplasmic reticulum, Golgi complex, secretory vesicles, cell surface). They offer the possibility of analyzing protein processing along this pathway, by studying the molecular intermediates accumulated in the mutants at the nonpermissive temperature. Compartmentalization of the proteins is influenced by cellular functions, as well as by their own peptide sequence of the molecules to be targeted. The physical structure of the wall is also important for the incorporation of mannoproteins into it. Factors affecting expression and externalization of heterologous proteins in yeasts are also reviewed.