In this paper, the design and test of different microwave antennas and sensors for breast cancer detection are presented. The sensors are designed and optimized to be used in direct skin contact, and for this purpose a specific breast phantom model is proposed. First, a miniaturized microstrip back-cavity Hilbert fractal antenna, operating in the ISM band (2.4–2.5 GHz), was designed. Then, this antenna was used to investigate the possibility of detecting the presence of breast tumors based on a narrowband frequency method that monitors the shift of the antenna frequency response. The antenna prototype was fabricated and tested in real in vivo measurement conditions on two different patients diagnosed with breast cancer. Measurement results have led after a comparison with the retro-simulation results of the structure to a more realistic breast model and to draw the limitations of this narrowband frequency method. As a time domain study seems to be more relevant, an UWB monopole antenna of dimensions 3 cm × 3 cm, to be used in direct contact with the breast model was designed. This antenna was optimized to both enhance the antenna/human body matching and to maximize the transfer of energy into the breast phantom by using a cavity, increasing by this way the detection potential. In order to improve the sensor’s directivity and enhance the electromagnetic field level inside the breast, a balanced antipodal Vivaldi antenna was also designed and optimized for a direct breast contact to operate in the 3.1–10.6 GHz band. A mono static and a bi static study in the time domain are finally proposed to investigate the presence of the tumor.