Surveys for transiting extrasolar planets, includingthe space-based missions CoRoT and Kepler, areexpected to detect hundreds of Jovian mass planets and tens ofterrestrial mass planets. For many of these newly discovered planets,the intervals between successive transits will be measured with anaccuracy of a few seconds. These timing measurementsallow for the detection of additional planets in the system (notnecessarily transiting), via their gravitational interaction with thetransiting planet. The transit time variations depend on the mass ofthe additional planet, and in some cases Earth-mass planets willproduce a measurable effect. When two or more planetstransit the same star, the densities of the planets can be estimatedfrom the photometry alone. I will review recent developments relatedto transits in multiple planet systems.