User-centric cell-free (UCCF) wireless networks have a range of distinguished characteristics, which can be exploited for meeting some challenges that the conventional cellular systems are hard to. This chapter is devoted to delivering the fundamentals of wireless communications in UCCF systems, including channel modeling and estimation, uplink (UL) detection, downlink (DL) transmission, and resource optimization. Specifically, the advantages of cell-free networking are examined in contrast to the conventional celluar systems. The global and location-aware distributed UL detection are explored in the principles of minimum mean-square error (MMSE) and brief propagation. Correspondingly, the global and distributed DL transmission schemes are designed based on the MMSE precoding. The optimization of both UL and DL is analyzed with respect to system design and resource-allocation. Furthermore, some challenges for the implementation of UCCF systems in practice are identified and analyzed.