This thesis describes the design and construction of a close range Passive Millimetre Wave (PMMW) scanning thermal imager. Whilst close range PMMW imager has previously been applied to concealed weapon detection at ranges of a few metres, the imager described herein is designed to focus on targets at a range of a few tens of centimetres. In particular, the main design aim was to produce higher resolution thermal maps suitable for medical imaging applications. Imaging at MMW frequencies offers greater penetration depths in lossy dielectric media than conventional infrared imagers, although there is an obvious trade-off in spatial resolution. The instrument consists of a total power radiometer operating at a centre frequency of 94 GHz. The input to the radiometer is provided by a quasi-optical focussing lens, designed using Gaussian Bean Mode theory. The observed scene is scanned by means of a rotating mirror and a translation table. Image acquisition timescales were of the order of a few minutes. Thermal calibration of the radiometer output was performed by recording the image of adjacent hot and cold reference loads with each line scan. In addition, the thermal transition between the calibration loads was used to measure the beam profile of the input optics and the Modulation Transfer Function (MTF) of the instrument. The imager has shown that it is possible to produce useful close range thermal images of the human body at MMW frequencies. The spatial resolution achieved was approximately 3 mm, with a thermal resolution of 0.4 K.