Photoacoustic tomography (PAT) is an imaging modality based on coupled physics of light and ultrasound. In PAT, an initial pressure distribution generated by absorption of an externally introduced light pulse is estimated from photoacoustic waves measured on the boundary of the imaged target. Solving this estimation problem relies on knowledge of the speed of sound in the medium. The speed of sound, however, is not know in practice. In this work, we study simultaneous estimation of initial pressure and speed of sound in PAT. We propose the use of multiple initial pressure distributions to alleviate the ill-posedness of this inverse problem. This is joint work with S. Arridge, B. Cox, F. Lucka, A. Pulkkinen and M. Suhonen.