  We present GLASS, the Generator for Large Scale Structure, a new code for the
simulation of galaxy surveys for cosmology, which iteratively builds a light
cone with matter, galaxies, and weak gravitational lensing signals as a
sequence of nested shells. This allows us to create deep and realistic
simulations of galaxy surveys at high angular resolution on standard computer
hardware and with low resource consumption. GLASS also introduces a new
technique to generate transformations of Gaussian random fields (including
lognormal) to essentially arbitrary precision, an iterative line-of-sight
integration over matter shells to obtain weak lensing fields, and flexible
modelling of the galaxies sector. We demonstrate that GLASS readily produces
simulated data sets with per cent-level accurate two-point statistics of galaxy
clustering and weak lensing, thus enabling simulation-based validation and
inference that is limited only by our current knowledge of the input matter and
galaxy properties.
