We present PAM, the Photoacoustic Mammoscope developed at the University of Twente, intended for initial retrospective clinical studies on subjects with breast tumours. A parallel plate geometry has been adopted and the breast will be gently compressed between a glass plate and a flat ultrasound detector matrix. Pulsed light (5 ns) from an Nd:YAG laser will impinge the breast through the glass plate in regions of interest; an appropriate number of the 590 elements of the detector matrix will be activated in succession to record photoacoustic signals. Three-dimensional image reconstruction employs a delay-and-sum beamforming algorithm. We discuss various instrumental aspects and the proposed imaging protocol. Performance studies of the ultrasound detector are presented in terms of sensitivity, frequency response and resolution. Details of the patient-instrument interface are provided. Finally some imaging results on well-characterized breast tissue phantoms with embedded tumour simulating inserts are shown.