Breast cancer is the most common cause of non-skin malignancy in women and a leading cause of female mortality. A potentially important strategy for reducing this menace is the detection at an early stage. However, conventional mammographic detection techniques have many drawbacks and often cause harm to patients. The invention of non-invasive and non-ionizing microwave technique, to reveal the internal structure of biological objects was a break through in the field of medical diagnostics. This book deals with the development of active microwave imaging techniques for breast cancer detection. When exposed to microwaves, breast tumor exhibits electrical properties that are significantly different from that of healthy breast tissues. The two approaches of active microwave imaging confocal microwave imaging with measured reflected signals and microwave tomographic imaging with measured scattered signals are addressed here.