Surveys for transiting extrasolar planets, including the space-based missions CoRoT and Kepler, are expected to detect hundreds of Jovian mass planets and tens of terrestrial mass planets. For many of these newly discovered planets, the intervals between successive transits will be measured with an accuracy of a few seconds. These timing measurements allow for the detection of additional planets in the system (not necessarily transiting), via their gravitational interaction with the transiting planet. The transit time variations depend on the mass of the additional planet, and in some cases Earth-mass planets will produce a measurable effect. When two or more planets transit the same star, the densities of the planets can be estimated from the photometry alone. I will review recent developments related to transits in multiple planet systems.