Heat-transfer considerations are critical in the design of multiple reaction microdevices. We present a new methodology for the design and fabrication of multiple reaction systems using the concept of heat integration. In our application, we use heat integration to minimize the overall power consumption in multiple-reaction device operation by optimizing component placement. Designs based on heat integration rely on the use of steady-state temperature gradients developed in the substrate by the actuation of a single reaction chamber to initiate several reactions at progressively lower temperatures. The use of thermal gradients in this manner places design constraints on component placement, but eliminates the need to heat each individual reaction chamber and actively control the temperature of those chambers, and eliminates the power requirement for active cooling. Complete thermal isolation of components in a device, on the other hand, requires higher power input for overall device cooling and local heating but, unlike designs based on heat integration, does not constrain geometry of reaction chambers or their placement.