A very natural model to study the information theoretic trade-off between sensing and communications in ISAC is the channel with states, which has been introduced in Chap.  2 . In most studies on channels with states, the goal is to maximize the channel capacity, without exploring the possibility and precision of estimating the channel state. Before the invention of ISAC, there have been a few studies in traditional information theory that exploits both data communications and state estimation. These studies have been exploited by the research community of ISAC.