Lactic acid bacteria are of major economic importance, as they occupy a key position in the manufacture of fermented foods. A considerable body of research is currently being devoted to the development of lactic acid bacterial strains with improved characteristics, that may be used to make fermentations pass of more efficiently, or to make new applications possible. Therefore, and because the lactococci are designated 'GRAS' organisms ('generally recognized as safe') which may be used for safe production of foreign proteins, detailed knowledge of homologous and heterologous gene expression in these organisms is desired. An overview is given of our current knowledge concerning gene expression in Lactococcus lactis. A general picture of gene expression signals in L. lactis emerges that shows considerable similarity to those observed in Escherichia coli and Bacillus subtilis. This feature allowed the expression of a number of L. lactis-derived genes in the latter bacterial species. Several studies have indicated, however, that in spite of the similarities, the expression signals from E. coli, B. subtilis and L. lactis are not equally efficient in these three organisms.