Here we investigated the feasibility of using microwave spectroscopy for characterization of normal and breast cancer cell lines cultured in vitro. Healthy non-tumorigenic, MCF-10A and breast cancer, MDA-MB-231, Hs578T, T47D and MCF-7 cell lines were electrically characterized using the open-ended coaxial probe technique from 200 MHz to 13.6 GHz. The dielectric constant, dielectric loss and conductivity between breast non-tumorigenic and breast cancer cells lines were analyzed and their differences determined. Our results showed that the four breast cancer cell lines analyzed exhibited higher dielectric properties when compared to healthy cells. Interestingly, we found that breast and colon cancer cells have different dielectric properties as well, thus suggesting that each type of cancer has a unique microwave signature. This study shows that microwave characterization of breast cancer cell lines is reliable with potential in biomedical applications such as designing electromagnetic models for detection of tumorous cells in healthy tissues.