Abstract: Clusters of galaxies are the largest gravitationally bound systems in the Universe. Due to their large masses (~1E15 suns), they distort the background galaxies images. The distortion (lensing) depends among other things on the angular distance to the galaxies versus redshift. The effect can be used to set limits to the Dark Energy equation of state w and even dw/dz. The exciting aspect is that the effect is purely geometrical. As such, it requires essentially no input physics to interpret the results. The approach is so obviously interesting that the fact it has not yet led to a definitive result indicates it must be difficult! We discuss our own group’s efforts and goals (http://cencosw.oamp.fr/DAFT/) and why the concept is difficult to carry out in practice. We also present one “fun” result based on using lensing and other techniques to measure the detailed mass distribution in a cluster. The mass distribution versus radius we find is such that the ratio of baryons to Dark Matter (DM) is not constant on large (1,000s of light years) scales. This result raises the specter that perhaps in our location of the Milky Galaxy the ratio of DM/baryons also deviates from the comic average.