Integrated waveguide meshes are the third evolutionary step in programmable photonics, and rely on the large-scale repetition of interferometric waveguide elements and phase actuators conceived and designed to implement a common hardware platform that enables the programming of arbitrary photonic circuit topologies and design parameters by suitable setting of its control signals. In contrast to linear feedforward-only programmable circuits, these circuits enable the synthesis of optical cavities, optical loops and feedbackward paths. Their improved versatility entails a paradigm shift regarding the development of application-specific photonic integrated circuits. Here, the long and costly test cycles can be potentially substituted by off-the-shelf ready-to-use circuits. This allows them to be applied to myriad systems requiring optical linear processing and dynamic configuration. This chapter introduces the basic design principles and the implementations of the most popular designs. It then provides a comparative analysis of alternative waveguide mesh arrangements and their performance evaluation.