Two of the most important problems in modern physics are (i) the incompatibility of quantum mechanics with general relativity, and (ii) the application of quantum mechanics at the macroscopic scale, where we believe objects are in definite states. This project will explore theories that yield 'intrinsic decoherence' via a breakdown of quantum mechanics at large scales. We will derive a theoretical framework, based on path integrals, in which quantitative calculations can be done. This will include 'gravitational decoherence' (an intrinsic decoherence mechanism proposed by R Penrose), and other intrinsic mechanisms which arise naturally in the new framework. Our predictive work will focus on experiments we will propose, involving a combination of optomechanical, superconducting, and spin systems. These have a well understood and often very low environmental decoherence, which in any experiment must be separated from intrinsic decoherence. Thus we will provide detailed predictions of both environmental and intrinsic decoherence rates, for real experimental systems. If intrinsic decoherence is found, this would provide an essential clue to a theory which might replace quantum mechanics. In another part of the work, we will study certain toy models intended to clarify how graviational decohrence works, including a 'spherical shell model for quantum gravity. The project will support the theoretical work, as well as funding two meetings bringing together theory and experiment.