Microwave photonics is now a mature research area, the first papers in the field having been published some fifty years ago. Some application areas, such as wireless over fiber, have enjoyed major commercial success, with annual sales revenues of > $100 m and systems installed worldwide. Other areas, such as optical beam-forming for antennas and optical filtering for microwave systems have delivered impressive research results but have yet to find widespread application. In this paper the systems enabling capabilities of microwave photonics will be reviewed and emerging technologies that could increase the range of applications for which microwave photonics offers an attractive solution will be described.