Microwave-based thermoacoustic tomography (TAT) and laser-based photoacoustic tomography (PAT) in a circular scanning configuration were both developed to image deeply seated lesions and objects in biological tissues. Because malignant breast tissue absorbs microwaves more strongly than benign breast tissue, cancers were imaged with good spatial resolution and contrast by TAT in human breast mastectomy specimens. Based on the intrinsic optical contrast between blood and chicken breast muscle, an embedded blood object that was 5 cm deep in the tissue was also detected using PAT at a wavelength of 1064 nm.