  I briefly discuss some attempts to construct a consistent modification to
General Relativity (GR) that might explain the observed late-time acceleration
of the universe and provide an alternative to dark energy. I mention the issues
facing extensions to GR, illustrate these with two specific examples, and
discuss the resulting observational and theoretical obstacles. This article
comprises an invited talk at the NASA workshop {\it From Quantum to Cosmos:
Fundamental Physics Research in Space}
