Abstract: For the ultrasound based biological imaging techniques, such as ultrasonic imaging, photo acoustic tomography (PAT) and magneto acoustic tomography (MAT), the spatial distribution of acoustic impedance, light absorption, optical parameters or electrical conductivity in the imaging tissue is always reconstructed under the assumption of a constant speed of sound (SOS) in tissues. However, the actual difference of SOS in different biological tissues may be up to 10%. This paper reviews the current methods to solve the problem of SOS inhomogeneity based on a brief introduction to the influence of SOS inhomogeneity on acoustic image reconstruction. Especially, the methods of restoring the distribution of SOS in the target tissue in PAT are overviewed including their advantages and disadvantages. Possible improvement in the future is also forecasted.