Massive galaxy clusters are important probes of structure formation and 
cosmology, and are also natural gravitational telescopes which can bring 
lensed primordial galaxies from the earliest epochs into comfortable reach 
for detailed studies. 
The CLASH project (Cluster Lensing And Supernova survey with Hubble) combines 
an HST Treasury program to obtain panchromatic (ACS+WFC3) imaging of 25 
carefully selected massive clusters, with other multi-wavelength observations 
and a large spectroscopic campaign with VLT/VIMOS. This program was designed 
to provide landmark progress in the investigation of the mass distribution of 
galaxy clusters, specifically the measurement of dark matter and total 
density profile shapes from a combination of independent probes (lensing, 
X-ray, and dynamics) with unprecedented accuracy, thus testing distinctive 
predictions of the LCDM cosmological model. I will describe the underlying 
methodology and present a selection of results in this area. Five powerful 
strong lensing systems were also included in the CLASH sample to detect 
the most distant magnified galaxies at the edge of the reionization epoch. 
I will show how that this strategy is proving to be an efficient way of 
unveiling galaxies out to z_phot~11 to date, providing the first estimate of 
the star-formation rate density at z~7-10, some 500 Myrs after the big bang. 
The third component of the project, the HST parallel search for Type-Ia 
supernovae well beyond z=1 designed to constrain the time dependence of 
the dark energy density, will not be covered in this talk.