To measure only photons that traveled through a volume of interest, ultrasound (US) is employed to encapsulate this bounded region from the characteristic focal zone of the transducer. The photons passing through the US focus will be frequency shifted or, in other words, phase modulated and thus tagged by the US. A variety of tagging strategies have been developed, along with different detection schemes to find the tagged photons in the sea of untagged photons. The tagged photons detected after exiting the medium provide information on the optical properties present in the vicinity of the volume of interest. When the tagging zone is placed in a tissue volume with high optical absorption compared with the rest of the object, less tagged photons will be detected. This in contrast with when the ultrasound focus is placed in the neighboring tissue. In this review we describe the progress made in AO imaging since the previous review by Wang10 in 2003: theoretical models, the detection methods to detect the tagged photons, and applications of AO imaging. For further reading, are view by Elson et al.11 is also focusing on the detection methods, scanning, and applications. They give, however, no equations from the models in the modeling section.