A new microwave imaging method is given for breast tumor detection using an ultrawideband (UWB) imaging system. By combining the time of arrival (TOA) measurements from different sensors, the presence and location of small malignant lesions can be identified. At each sensor, the generalized sequence CLEAN (GS-CLEAN) algorithm is proposed to resolve the impulse response (IR) components into bins smaller than the duration of the sounding pulse. This novel deconvolution technique, working well with largely continuous image content, also solves an inverse problem to estimate the unknown tumors' morphological/dielectric properties and delay intervals. Subsequently, the TOA data fusion step is applied to obtain an estimate of the locations of dominant scattering sources such as cancerous tumors. We also apply the entropy-based approaches to collectively estimate the tumor morphological and electrical features based on the information acquired at various antenna elements. Preliminary two-dimensional (2-D) analysis and simulations demonstrate that the system developed is feasible to detect small breast malignant masses using this approach.