This chapter delves into the principles, architectures, and applications of Joint Communications and Sensing (JCAS) systems, a transformative technology central to 6G networks. It begins by examining the convergence of radar and communication systems, highlighting the potential of JCAS to optimize spectral efficiency, reduce hardware complexity, and enable dual functionality through shared signals. Key sections explore architecture models, including monostatic, bistatic, and multistatic setups, alongside innovations in millimeter-wave and sub-terahertz hardware design, such as integrated 60 GHz transceivers. Practical applications in areas such as autonomous vehicles, Industry 4.0, and healthcare are analyzed, emphasizing sensing-assisted communications and real-time environmental awareness. Advanced waveform design and optimization techniques are discussed in detail, focusing on achieving trade-offs between communication reliability and sensing accuracy. The chapter underscores the critical role of JCAS in shaping future 6G networks and outlines ongoing challenges in hardware integration and standardization.