Spintronics is a multidisciplinary field focused on the manipulation of spin degrees of freedom in solid-state systems. This review discusses current advancements and established results in spintronics, emphasizing the generation of spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport because spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors detail spin decoherence mechanisms in metals and semiconductors and apply various theories of spin injection and spin-polarized transport to hybrid structures relevant to spin-based devices and material studies. Experimental work is reviewed with a focus on projected applications, where external electric and magnetic fields and light illumination will be used to control spin and charge dynamics to create new functionalities not feasible with conventional electronics. The review covers the history and background of spintronics, including spin-polarized transport and magnetoresistive effects, spin injection and optical orientation, and the generation of spin polarization. It discusses the mechanisms of spin relaxation, such as the Elliott-Yafet, D'yakonov-Perel', Bir-Aronov-Pikus, and hyperfine-interaction mechanisms, and their implications for spin relaxation in metals and semiconductors. The review also explores spintronic devices and applications, including spin-polarized transport, materials considerations, spin filters, spin diodes, spin transistors, and spin qubits in semiconductor nanostructures. It concludes with an outlook on future prospects in spintronics. Spintronics benefits from emerging materials such as ferromagnetic semiconductors, organic semiconductors, high-temperature superconductors, and carbon nanotubes, which can bring novel functionalities to traditional devices. Fundamental studies are needed before the potential of spintronic applications is fully realized. The review also discusses the spin-valve effect, giant magnetoresistance (GMR), and spin-transfer torque, which are important for nonvolatile memory applications. The review highlights the importance of spin-polarized materials for large magnetoresistive effects and the role of spin-orbit coupling in spin orientation. It also discusses the generation of nonequilibrium spin polarization through transport, optical, and resonance methods, and the detection of spin polarization through spin accumulation and spin-charge coupling. The review concludes with a discussion of the Hanle effect and its implications for spin polarization in semiconductors.Spintronics is a multidisciplinary field focused on the manipulation of spin degrees of freedom in solid-state systems. This review discusses current advancements and established results in spintronics, emphasizing the generation of spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport because spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors detail spin decoherence mechanisms in metals and semiconductors and apply various theories of spin injection and spin-polarized transport to hybrid structures relevant to spin-based devices and material studies. Experimental work is reviewed with a focus on projected applications, where external electric and magnetic fields and light illumination will be used to control spin and charge dynamics to create new functionalities not feasible with conventional electronics. The review covers the history and background of spintronics, including spin-polarized transport and magnetoresistive effects, spin injection and optical orientation, and the generation of spin polarization. It discusses the mechanisms of spin relaxation, such as the Elliott-Yafet, D'yakonov-Perel', Bir-Aronov-Pikus, and hyperfine-interaction mechanisms, and their implications for spin relaxation in metals and semiconductors. The review also explores spintronic devices and applications, including spin-polarized transport, materials considerations, spin filters, spin diodes, spin transistors, and spin qubits in semiconductor nanostructures. It concludes with an outlook on future prospects in spintronics. Spintronics benefits from emerging materials such as ferromagnetic semiconductors, organic semiconductors, high-temperature superconductors, and carbon nanotubes, which can bring novel functionalities to traditional devices. Fundamental studies are needed before the potential of spintronic applications is fully realized. The review also discusses the spin-valve effect, giant magnetoresistance (GMR), and spin-transfer torque, which are important for nonvolatile memory applications. The review highlights the importance of spin-polarized materials for large magnetoresistive effects and the role of spin-orbit coupling in spin orientation. It also discusses the generation of nonequilibrium spin polarization through transport, optical, and resonance methods, and the detection of spin polarization through spin accumulation and spin-charge coupling. The review concludes with a discussion of the Hanle effect and its implications for spin polarization in semiconductors.