The thermal properties of a high-power vertical-external-cavity surface-emitting laser (VECSEL) are studied experimentally, focusing on the generation, distribution, and removal of excess heat under extreme pumping conditions. Different heat-spreading and heat-transfer approaches are analyzed. The performance of the device is optimized yielding a maximum emitted power beyond 70W from a single spot. Finally, the potential for power-scaling in VECSELs and its restrictions are examined.