The expression of a gene or a set of genes from one organism in a different species is known as "heterologous expression." In actinomycetes, prolific producers of natural products, heterologous gene expression has been used to confirm the clustering of secondary metabolite biosynthetic genes, to analyze natural product biosynthesis, to produce variants of natural products by genetic engineering, and to discover new compounds by screening genomic libraries. Recent advances in DNA sequencing have enabled the rapid and affordable sequencing of actinomycete genomes and revealed a large number of secondary metabolite gene clusters with no known products. Heterologous expression of these cryptic gene clusters combined with comparative metabolic profiling provides an important means to identify potentially novel compounds. In this chapter, the methods and strategies used to heterologously express actinomycete gene clusters, including the techniques used for cloning secondary metabolite gene clusters, the Streptomyces hosts used for their expression, and the techniques employed to analyze their products by metabolic profiling, are described.