The threat in modern life necessitates the use of security systems in many areas. Systems, whether manual search or automated, need to be able to detect concealed munitions beneath clothing and in baggage. Systems which scan people, unlike baggage, must be safe to avoid damaging those who must be repeatedly scanned. Passive millimeter wave (mmw) systems have the ability to scan people through clothing to detect concealed objects without irradiating the individual. The performance of such systems is dependent on operating frequency, which is a trade-off between resolution, clothing transmission, and material visibility. Transmission and reflection spectra of clothing, skin, and other materials which may be worn under clothing, over the frequency range 60 to 500 GHz, are presented with their implications for operating frequency. The practicalities of imaging are discussed, the differences between the indoor and outdoor situation highlighted, and the limitations of the indoor case described. Imagery of persons with concealed objects, obtained using DERA's MITRE 94 GHz mmw imager, are presented.