  We present the results of the GREAT08 Challenge, a blind analysis challenge
to infer weak gravitational lensing shear distortions from images. The primary
goal was to stimulate new ideas by presenting the problem to researchers
outside the shear measurement community. Six GREAT08 Team methods were
presented at the launch of the Challenge and five additional groups submitted
results during the 6 month competition. Participants analyzed 30 million
simulated galaxies with a range in signal to noise ratio, point-spread function
ellipticity, galaxy size, and galaxy type. The large quantity of simulations
allowed shear measurement methods to be assessed at a level of accuracy
suitable for currently planned future cosmic shear observations for the first
time. Different methods perform well in different parts of simulation parameter
space and come close to the target level of accuracy in several of these. A
number of fresh ideas have emerged as a result of the Challenge including a
re-examination of the process of combining information from different galaxies,
which reduces the dependence on realistic galaxy modelling. The image
simulations will become increasingly sophisticated in future GREAT challenges,
meanwhile the GREAT08 simulations remain as a benchmark for additional
developments in shear measurement algorithms.
