Photoacoustic (PA) imaging is a biomedical imaging technique that uses the PA effect to generate in vivo and ex vivo images of biological tissue. Intrinsic and exogenous contrast agents transfer laser-generated energy in acoustic emissions, which carry information about the absorption property inside tissues and are detected by an ultrasound transducer. Due to strong optical absorption and scattering, the penetration depth of visible light is limited to a few centimetres. To maximise the penetration depth, near-infrared (NIR) light is used. However, the endogenous contrast agents, like haemoglobin or melanin, are rather poor absorbers at these wavelengths, so exogenous contrast agents are introduced to overcome this problem. The advantages of these contrast agents are a contrast and a resolution improvement at a high penetration depth.In this thesis the contrast agent indocyanine green (ICG) is characterized by investigating the absorption properties and the long-time stability. Furthermore, different concentrations of ICG in different solvents are investigated to determine the most suitable combination of these for photoacoustic imaging. ICG was used in different phantom and biological samples during PA tomography.An albumin solution as a solvent led to a stabilization of the ICG over the first few days and to a maximum absorption at about 790 nm, whereas in a sodium chloride solution the absorption of ICG decreased rapidly. It was possible to image some bile ducts of a mouse liver and the blood vessel structure of a mouse brain using ICG as a contrast agent.