ISAC needs to achieve two goals, communications and sensing. Although communications and sensing are highly compatible (otherwise, we do not need to study how to integrate them in the same waveform), they still have many subtle differences that result in conflict in their performances. Therefore, the communication-sensing performance trade-off is a key topic for the fundamental performance bound of ISAC systems. In this chapter, we will study the communication-sensing trade-off from the viewpoint of signal processing. In the next two chapters, from the aspects of information theoretic analysis and electromagnetic field.