Powerful electronic design automation tools have enabled the rapid development of electronic Integrated Circuits (ICs). Similar to electronic ICs, silicon photonics technology has sufficiently matured, and large-scale photonic circuits can now be implanted into a single chip. Design tools have also evolved from primary devices to complex photonic circuits. In this paper, we review the current state of photonic design automation in terms of device modeling methods and circuit simulation methodologies, and compare the photonics design flow with mature electronic design automation design flows. Key challenges and opportunities are also discussed.