  Radar and communications (R&C) as key utilities of electromagnetic (EM) waves
have fundamentally shaped human society and triggered the modern information
age. Although R&C have been historically progressing separately, in recent
decades they have been converging towards integration, forming integrated
sensing and communication (ISAC) systems, giving rise to new, highly desirable
capabilities in next-generation wireless networks and future radars. To better
understand the essence of ISAC, this paper provides a systematic overview on
the historical development of R&C from a signal processing (SP) perspective. We
first interpret the duality between R&C as signals and systems, followed by an
introduction of their fundamental principles. We then elaborate on the two main
trends in their technological evolution, namely, the increase of frequencies
and bandwidths, and the expansion of antenna arrays. We then show how the
intertwined narratives of R&C evolved into ISAC, and discuss the resultant SP
framework. Finally, we overview future research directions in this field.
