Clusters of galaxies are among the richest astrophysical objects. To truly understand them, we need a detailed study of the relationship between observables and the underlying dark matter distribution. Gravitational Lensing is a direct probe of dark matter. I will talk about using lensing to map the mass distribution of galaxy clusters. In my thesis I have developed a new mass reconstruction technique "Particle Based Lensing (PBL)". This method optimally combines lensing information of varying signal-to-noise, and makes no assumptions about the relationship between mass and light. I have applied PBL to several clusters, including The super-cluster A901/902 and A1689. For A901/902, I have measured the ellipticity of the distribution of light and dark matter. A1689 is one of the richest clusters known, and has significant substructure at the core. I find that the dark matter distribution is significantly clumpier than indicated by X-ray maps of the gas. I will also describe an ongoing project involving a sample of 20 clusters to calibrate the mass-temperature relation. I aim to use this sample to do strong+weak lensing analysis and compare the resulting dark matter distribution to the gas distribution from X-rays and SZ effect.