- How galaxies build-up and evolve at early times is one of the most fundamental questions in extragalactic cosmology. While well established constraints exist on how this build-up occurs from rest-frame UV studies, an equally important part of the story - the build-up of stellar mass - is far less well understood. Although we have some results on the stellar mass at the brightest magnitudes (like the non-evolution of the specific SFR), the stellar masses of lower luminosity galaxies at high redshift remain essentially unknown. This is a major gap in our knowledge since lower luminosity galaxies dominate the SFR density, UV density, and likely provide the photons that reionize the universe. The only way of obtaining data on these faint sources at high redshift is with Spitzer, but with Spitzer one is faced with serious S/N limitations. To surmount these challenges, we propose to take advantage of gravitational lensing and a large sample of ~200 highly magnified low-luminosity galaxies expected to be found in the large 524-orbit, 25-cluster CLASH MCT program. We propose to obtain deep IRAC 3.6 and 4.5 micron observations over the full suite of CLASH clusters. Since 17 clusters from the program already have deep data, we are requesting data for the final eight. The additional data are important to ensure that these magnified samples remain sizeable after subdividing by redshift and luminosity. This ICLASH program will allow us to robustly derive stellar masses and do stellar population modelling at z>4, determine the stellar mass density at z~7-8 to faint limits, and add significance confidence to z~10 identifications in our program. We waive our right to any proprietary period for this important data set.