Galaxy clusters are fantastic laboratories to study the formation of structures in the Universe, to test our cosmological models, and to infer the properties of dark matter. Since many clusters show strong and weak lensing effects, we can use these to infer the cluster mass, thus obtaining precious information on the distribution of the luminous and dark matter. This is especially important since gravitational lensing models are known to be free from many of the systematic effects that plague the traditional mass-estimate methods. In this talk I will present state-of-art mass models of massive intermediate-redshift clusters studied with the HST imaging and with the VLT spectroscopy as part of the CLASH survey. I will discuss the results obtained through these efforts and show the implications for the Cold Dark Matter paradigm. I will also demonstrate how strong gravitational lensing allows us to obtain robust measurements of cosmological parameters from individual measurements in well-constrained galaxy clusters.