This chapter provides a comprehensive overview of direct current (DC) networks, encompassing both low-power and high-power applications within a unified framework. Low-power applications include DC microgrids—localized distribution systems that integrate renewable energy sources, energy storage devices, and controllable loads—commonly found in residential areas, commercial buildings, university campuses, and electric vehicle charging stations. On the other end of the spectrum, high-power applications are represented by multiterminal high-voltage direct current (MT-HVDC) transmission systems, which are essential components in the development of super-grids and international energy hubs. The chapter emphasizes the growing relevance of DC technology in modern power systems due to its efficiency, flexibility, and suitability for integrating distributed energy resources. Additionally, this chapter introduces the DC power flow problem as the foundational model for analyzing the steady-state operation of such networks. While simpler than its AC counterpart due to the absence of reactive power and frequency dynamics, the DC power flow problem remains nonlinear and poses significant challenges when incorporated into optimization frameworks. This motivates the study and development of specialized solution techniques, as explored throughout the book. The formulation of the power flow problem, its mathematical structure, and its role as a constraint in various optimal power flow (OPF) formulations are discussed in detail. By establishing the theoretical and practical context, this chapter lays the groundwork for the advanced methodologies presented in the subsequent chapters.