Gravitational lensing is a very powerful tool to probe the distribution of dark matter on the scale of galaxies and galaxy clusters (see e.g. the wikipedia article for basic concepts and references). Strong and weak lensing studies of massive galaxy clusters have given us unique insights on the dark matter density profile of these systems from the core to the outskirts, providing a crucial test of cosmological structure models. In central regions of massive objects, strong lensing can significantly magnify background sources allowing the discovery and detailed spectroscopic studies of primordial galaxies which would otherwise  escape detection (gravitational telescopes).
To enable a landmark progress in this field, we have initiated a new project, CLASH (Cluster Lensing And Supernova survey with Hubble), which combines a 524-orbit HST Treasury program (PI: M.Postman) to obtain panchromatic 16-filter (ACS+WFC3) imaging of 25 massive clusters, with a VLT Large Programme (PI: P.Rosati) to conduct a VIMOS panchromatic spectroscopic survey of the 14 southern clusters from this sample.

Several thesis projects are available from the VLT-CLASH data set, which include:
i) the study of cluster mass distributions (dark matter and baryons) from a combination of dynamical, lensing and X-ray methods;
ii) the study the galaxy populations from the spectro-photometric information on thousands of cluster members over the entire cluster volume out to the surrounding field;
iii) the identification and study of primordial magnified galaxies at z>6.

For more information on the CLASH project see the Postman et al. paper and this site.
Specifically on the CLASH-VLT project see this ESO Messenger article (Dec 2014) on the CLASH-VLT program.

Examples of recent CLASH observations

With current data from HST and VLT, these studies can be extended at out to redshift z~1 and beyond (see below).