Several dielectric spectroscopy studies reported in the literature over the past 20 years suggest that the dielectric-properties contrast between malignant and normal breast tissue is greater than 2:1 in the radio and microwave frequency range. This contrast can be attributed to increased protein hydration and vascularization of malignant tumors. Recognition of the potential diagnostic value of such a contrast is one of the primary motivating factors for the development of non-ionizing electromagnetic imaging technologies for breast cancer detection. In this talk, theoretical principles and experimental techniques for an emerging ultra-wideband microwave breast imaging modality will be presented. Space-time microwave imaging involves transmitting low-power, short-duration microwave signals into the breast from multiple spatial locations using a physical or synthetic array of ultra-wideband antennas placed near the surface of the breast. the backscattered signals are recorded and processed using on of several techniques currently under investigation. For example, a space-time beamforming technique may be applied to focus the backscattered signals using both temporal and spatial dimensions. Scanning the beamformer to different locations generates an image of backscattered signal energy as a function of position in the three-dimensional breast volume. Malignant tumors exhibit large microwave scattering cross-sections relative to comparably sized heterogeneity in normal breast tissue. Therefore, a localized region of the image associated with a large backscatter energy level indicates the presence of a malignant tumor. the feasibility of space-time microwave imaging is demonstrated theoretically using anatomically realistic complementary space-time microwave hyperthermia technique for treating breast cancer will also be presented.