There is a long history of attempts to modify General Relativity (GR). In the last decade, in particular, the subject has gained much momentum as a result of the observation that the universe is currently accelerating.
Indeed, such an acceleration could be explained by a weakening of gravity on large scales (relative to GR prediction), or by selfinduced acceleration. Such an explanation, however, does not eliminate the cosmological constant problem: in the specific example of massive gravity  a modification of GR in which the graviton has a mass m  it rears its head in the requirement that m must be small. However, the way in which this might happen can be more "natural".
In this workshop we plan to discuss different modifications of GR, from galileons to massive bigravity, focusing on aspects of their construction, mathematical consistency, and predictions.
