The development of quantum sensors is a major component of emerging quantum technologies. As our understanding of measurement in quantum mechanics advanced it became clear that there would be new opportunities for quantum sensors that could out perform their classical counterparts. Optical sensors have long been a key sector of the precision sensor industry and quantum optical sensors are a natural extension of this, especially for remote sensing. The recent progress in machine learning has led to a new kind of smart-sensor and these ideas can be readily applied to quantum optical sensors.