Thermal stimulation is a rich, emotive and salient feedback channel that is well suited to HCI, but one that is yet to be fully investigated. Thermal feedback may be suited to environments that are too loud for audio or too bumpy for vibrotactile feedback. This paper presents two studies into how well users could detect hot and cold stimuli presented to the fingertips, the palm, the dorsal surface of the forearm and the dorsal surface of the upper arm. Evaluations were carried out in static and mobile settings. Results showed that the palm is most sensitive, cold is more perceivable and comfortable than warm and that stronger and faster-changing stimuli are more detectable but less comfortable. Guidelines for the design of thermal feedback are outlined, with attention paid to perceptual and hedonic factors.