  We present an overview of the "KIFF" project, which provides ultra-deep
Ks-band imaging of all six of the Hubble Frontier Fields clusters Abell 2744,
MACS-0416, Abell S1063, Abell 370, MACS-0717 and MACS-1149. All of these fields
have recently been observed with large allocations of Directors' Discretionary
Time with the HST and Spitzer telescopes covering 0.4 < lambda < 1.6 microns
and 3.6--4.5 microns, respectively. VLT/HAWK-I integrations of the first four
fields reach 5-sigma limiting depths of Ks~26.0 (AB, point sources) and have
excellent image quality (FWHM ~ 0."4). Shorter Keck/MOSFIRE integrations of the
MACS-0717 (MACS-1149) field better observable in the north reach limiting
depths Ks=25.5 (25.1) with seeing FWHM ~0."4 (0."5). In all cases the Ks-band
mosaics cover the primary cluster and parallel HST/ACS+WFC3 fields. The total
area of the Ks-band coverage is 490 arcmin^2. The Ks-band at 2.2 microns
crucially fills the gap between the reddest HST filter (1.6 micron ~ H-band)
and the IRAC 3.6 micron passband. While reaching the full depths of the
space-based imaging is not currently feasible from the ground, the deep Ks-band
images provide important constraints on both the redshifts and the stellar
population properties of galaxies extending well below the characteristic
stellar mass across most of the age of the universe, down to, and including,
the redshifts of the targeted galaxy clusters (z < 0.5).
