Terahertz (1012 Hz) pulsed imaging is a totally non-destructive and non-ionising imaging modality and thus potential applications in medicine are being investigated. In this paper we present results using our hand-held terahertz probe that has been designed for in vivo use. In particular, we use the terahertz probe to perform reflection geometry in vivo measurements of human skin. The hand-held terahertz probe gives more flexibility than a typical flat-bed imaging system, but it also results in noisier data and requires existing processing methods to be improved. We describe the requirements and limitations of system geometry, data acquisition rate, image resolution and penetration depth and explain how various factors are dependent on each other. We show how some of the physical limitations can be overcome using novel data processing methods.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.