PUFs (Physical Unclonable Functions) have been proposed as a cost-effective solution to provide a root of trust for electronic devices which exploit intrinsic process variability. They generate identification signatures and keys only when the devices are turned on, avoiding the storage of sensitive information in memories that could be targeted by attacks. Although PUFs have many perceived advantages, they also have disadvantages such as sensitivity to temperature. Indeed their behaviour can be affected by the fact that high temperatures can accelerate permanent and transient phenomena, such as aging and transistor switching speed. In this paper we show the effects of externally induced heat on the functioning of Ring Oscillators (ROs), which form the basis of RO-PUFs. Moreover, we discuss the feasibility of temperature attacks on PUFs.