Microwave imaging for medical applications has been of interest for many years. Recently, microwave imaging for breast cancer detection has gained attention due to advances in imaging algorithms, microwave hardware and computational power. The breast is relatively translucent to microwaves, accessible for imaging, and there appears to be a significant electromagnetic property contrast between tumors and healthy tissues. Therefore, breast imaging may be the first clinically viable application of microwave imaging. This paper reviews recent developments in passive, hybrid, and active approaches to microwave breast cancer detection.