The present disclosure is drawn to temperature-cycling microfluidic devices. In one example, a temperature-cycling microfluidic device can include a driver chip having a top surface and a heat exchange substrate having a top surface coplanar with the top surface of the driver chip. A fluid chamber can be located on the top surface of the driver chip. A first and second microfluidic loop can have fluid driving ends and fluid outlet ends connected to the fluid chamber and can include portions thereof located on the top surface of the heat exchange substrate. A first and second fluid actuator can be on the driver chip. The first and second fluid actuators can be associated with the fluid driving ends of the first and second microfluidic loops, respectively, to circulate fluid through the first and second microfluidic loops.