Transduction mechanisms involving thermal phenomena play a central role in a wide range of microelectromechanical systems (MEMS) applications. An overview of a subset of thermal issues in MEMS technology is presented, including a discussion of traditional and emerging applications for microscale thermal systems. Issues relating to fundamental limitations and opportunities in thermal microsystems are presented. The use of thermal phenomena in three specific microsystems is reviewed, namely microhotplate chemical sensors, microfluidic systems, and electrothermal micromotors. Future directions in microscale and nanoscale thermal systems are presented.