This paper discusses the development of programmable photonic circuits, which can be programmed in software to perform a wide range of functions on a single chip using waveguides, tunable beam couplers, and optical phase shifters. The authors review recent advancements in building blocks, circuit architectures, electronic control, and new programming strategies. They highlight applications in linear matrix operations, quantum information processing, and microwave photonics, and discuss how these chips can significantly accelerate future photonic circuit development. Programmable photonic circuits are more general than application-specific photonic integrated circuits (ASPICs) and can be deployed in various applications. They enable the manipulation of light in a flexible and reconfigurable manner, allowing for real-time adaptation to changing problems. The paper explores two main architectures: forward-only meshes, where light flows in one direction, and recirculating meshes, which allow light to be routed in loops and back to input ports. The paper also discusses the technology stack required for programmable photonic circuits, including driver and monitor electronics, control loops, configuration software, and packaging. It covers the key components such as tunable couplers and phase shifters, which are essential for the functionality of these circuits. The paper highlights the importance of low optical insertion loss and low electrical power consumption in these components. The authors also discuss the potential applications of programmable photonic circuits in areas such as microwave photonics, optical beamforming, and sensing. They emphasize the importance of reconfigurability and adaptability in these circuits, which can significantly reduce product development time and enable new applications in functional optical systems. The paper concludes by discussing the potential of programmable photonic circuits to change the way people use coherent light to manipulate information, similar to how programmable electronics have evolved. It suggests that a complete technology stack, including electronics, packaging, and software layers, is needed to realize the full potential of programmable photonic circuits. The authors also highlight the importance of standardization and the need for a programming infrastructure to support the development of these circuits.This paper discusses the development of programmable photonic circuits, which can be programmed in software to perform a wide range of functions on a single chip using waveguides, tunable beam couplers, and optical phase shifters. The authors review recent advancements in building blocks, circuit architectures, electronic control, and new programming strategies. They highlight applications in linear matrix operations, quantum information processing, and microwave photonics, and discuss how these chips can significantly accelerate future photonic circuit development. Programmable photonic circuits are more general than application-specific photonic integrated circuits (ASPICs) and can be deployed in various applications. They enable the manipulation of light in a flexible and reconfigurable manner, allowing for real-time adaptation to changing problems. The paper explores two main architectures: forward-only meshes, where light flows in one direction, and recirculating meshes, which allow light to be routed in loops and back to input ports. The paper also discusses the technology stack required for programmable photonic circuits, including driver and monitor electronics, control loops, configuration software, and packaging. It covers the key components such as tunable couplers and phase shifters, which are essential for the functionality of these circuits. The paper highlights the importance of low optical insertion loss and low electrical power consumption in these components. The authors also discuss the potential applications of programmable photonic circuits in areas such as microwave photonics, optical beamforming, and sensing. They emphasize the importance of reconfigurability and adaptability in these circuits, which can significantly reduce product development time and enable new applications in functional optical systems. The paper concludes by discussing the potential of programmable photonic circuits to change the way people use coherent light to manipulate information, similar to how programmable electronics have evolved. It suggests that a complete technology stack, including electronics, packaging, and software layers, is needed to realize the full potential of programmable photonic circuits. The authors also highlight the importance of standardization and the need for a programming infrastructure to support the development of these circuits.