This chapter aims to provide an overview of optical microsystem technologies and their applications in biology. First, the general concepts of light, optical instrumentation, and basic principles of light-tissue interaction are summarized. A few key categories of optical microsystems are subsequently introduced including MOEMS devices, optofluidic devices, micro-flow cytometers, and wearable optical technology. Finally, preferences and metrics of biophotonic microsystems, challenges and future perspectives are discussed. Within this chapter, we hope to provide a general development process in the optical microsystem field.