The chapter discusses the complexity in strongly correlated electronic systems, particularly focusing on transition metal oxides (TMOs) and high-temperature superconductors (HTSCs). It highlights that these materials exhibit non-homogeneous states at the nanoscale due to the simultaneous activity of multiple physical interactions (spin, charge, lattice, and orbital). This complexity leads to interesting phenomena such as colossal magnetoresistance (CMR) and high-temperature superconductivity (HTSC). The chapter emphasizes the importance of understanding these complex systems in the broader context of complexity, where the properties of individual particles do not fully explain the behavior of the system as a whole. It also discusses the role of quenched disorder and phase competition in generating inhomogeneous patterns and their impact on material properties. The chapter concludes by exploring the potential applications of these complex behaviors, such as in field-effect transistors and ferroelectric devices, and the need for advanced theoretical and computational methods to understand and control these systems.The chapter discusses the complexity in strongly correlated electronic systems, particularly focusing on transition metal oxides (TMOs) and high-temperature superconductors (HTSCs). It highlights that these materials exhibit non-homogeneous states at the nanoscale due to the simultaneous activity of multiple physical interactions (spin, charge, lattice, and orbital). This complexity leads to interesting phenomena such as colossal magnetoresistance (CMR) and high-temperature superconductivity (HTSC). The chapter emphasizes the importance of understanding these complex systems in the broader context of complexity, where the properties of individual particles do not fully explain the behavior of the system as a whole. It also discusses the role of quenched disorder and phase competition in generating inhomogeneous patterns and their impact on material properties. The chapter concludes by exploring the potential applications of these complex behaviors, such as in field-effect transistors and ferroelectric devices, and the need for advanced theoretical and computational methods to understand and control these systems.