Cyber Physical Systems (CPSs) integrate control,communication and computation technologies to facilitate close interactions between human being and physical world.CPS is widely recognized as one of the revolutional technologies for smart cities,smart grid,intelligent manufacturing and so on.In all of these applications,due to potential vulnerability to cyber attacks,CPS security is one of the key design concerns and has drawn increasing research attention recently.In this paper,we provide a review of recent studies on CPS security.We first introduce several types of security threats that are able to cause severe damage to any target CPS if not well protected.We then present a survey of recent literature on attack analysis,detection and defeating strategies,and discuss a list of research challenges in this area.