The completion of the genomic sequences of many protozoan pathogens of humans, including species of Leishmania, Trypanosoma and Plasmodium, provide new approaches to study the pattern of gene expression during differentiation and development. Leishmania are a major public health risk in many countries and cause a wide spectrum of clinical disease referred to as leishmaniasis. The Leishmania life cycle consists of two morphologically distinct stages: intracellular amastigotes that reside in the phagolysosome of mammalian macrophages, and extracellular promastigotes that reside within the gut of the sandfly vector. DNA microarray analysis is a powerful method to study global gene expression in terms of quantitation of mRNA levels. This review discusses the application of DNA microarray technology to study the pattern of global gene expression of Leishmania promastigote and amastigote life stages. Results from several studies show that, overall, there is a surprisingly low level of differentially expressed genes, ranging from 0.2% to 5% of total genes, between the amastigote and promastigote life stages. Thus, the Leishmania genome can be considered to be constitutively expressed with a limited number of genes showing stage-specific expression. Comparative genomic analyses of gene expression levels between Leishmania major and Leishmania mexicana show that the majority of differentially expressed genes between amastigotes and promastigotes are species specific with relatively few differentially expressed genes in common between these two Leishmania species. Quantitative proteomic analysis of Leishmania relative protein expression shows there is a weak correlation to gene expression. Therefore, Leishmania protein expression levels are likely regulated at the level of translation or by post transcriptional mechanisms, and differential protein modifications may be more important in development than the regulation of gene expression.