Abstract: Single molecule technologies generally require sensitive optical detection and the ability to operate at multiple temperatures simultaneously in different parts of the instrument. The system for controlling the temperature of a microfluidic device and methods for controlling the temperature of sequencing reactions includes a chamber for receiving a microfluidic device, a heating control device in fluid communication with the chamber for delivering a heated fluid to the chamber to heat the microfluidic device, and a cooling control device in liquid communication with the chamber for delivering a cooled fluid to the chamber to cool the microfluidic device. A temperature control unit in liquid communication with a cooling element and/or a heating element are used to regulate temperature of sequencing substrates and objective lenses for optical detection of sequencing reactions.