Abstract: Photoacoustic imaging, based on photoacoustic effect, is a new type of imaging technology. It has the advantages of both high contrast of optical imaging and high penetrability of ultrasound imaging. Photoacoustic imaging molecules consist of intrinsic chromophores and exogenous contrast agents. With the use of exogenous contrast agents, this modality has shown potential for molecular imaging, which has broad prospect and great importance in biomedical research. In this paper, we have reviewed exogenous contrast agents for molecular photoacoustic imaging, such as small-molecule organic dyes, noble metal nanoparticles, carbon nanostructures, organic polymer nanoparticles, genetically encoded chromophores, copper and iron compounds, and semiconducting polymer nanoparticles. Their physical and chemical characteristics and synthesis methods are talked about. Ligand molecules such as small molecules, peptides, affibodies, aptamers and antibodies are introduced. The future developing researches are also prospected.