Among the secondary metabolites produced by plants, alkaloids such as morphine have high bioactivity, and they are used as raw materials for pharmaceuticals such as analgesics. However, many of them are present only at very low levels in plants, rendering their production from extracted raw materials relatively expensive. Thus, there is a need to develop inexpensive production methods. Unfortunately, because of their complex structure, chemical synthesis methods are not practical for the production of alkaloids. In recent years, comprehensive genetic analysis of secondary metabolite-producing plants has revealed details of the metabolic pathways by which various alkaloids are produced. The production of plant secondary metabolites by microorganisms using synthetic biology has attracted considerable attention; on the basis of these findings, plant alkaloid production by microbial fermentation has been attempted. In this chapter, we outline our knowledge of biosynthesis of reticuline, an important intermediate of isoquinoline alkaloids, and the current status and prospects of microbial reticuline and isoquinoline alkaloid fermentation.