Intracavity Phase Interferometry (IPI) makes use of the the frequency comb of a mode-locked laser to perform precise phase measurements. By interfering two pulses in a mode-locked laser, IPI achieves extremely high measurement quality in a simple tabletop setup for a wealth of applications. This text summarizes work on intracavity interferometers and introduces two optical parametric oscillator IPI setups. It describes the design and numerical simulation of optical parametric oscillators and their application as intracavity interferometers. It discusses several realizations of the concept demonstrating ultra-sensitive phase measurements. Finally some ground work for future intracavity interferometers is discussed. This includes the potential use of semiconductor lasers to pump an OPO.