The article provides an overview of spintronics, a field that focuses on the active control and manipulation of spin degrees of freedom in solid-state systems. It reviews the current status of spintronics, including recent advances and established results, with a primary emphasis on the fundamental physical principles underlying carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. The authors discuss spin decoherence mechanisms in metals and semiconductors in detail and apply various theories of spin injection and spin-polarized transport to hybrid structures relevant to spin-based devices and fundamental studies of material properties. Experimental work is reviewed, with a focus on projected applications where external electric and magnetic fields and quantum effects are harnessed to control and manipulate charge dynamics, creating new functionalities not feasible with conventional electronics. The article also covers the history and background of spin-polarized transport and magnetoresistive effects, as well as the techniques for generating spin polarization, such as optical orientation and electrical spin injection.The article provides an overview of spintronics, a field that focuses on the active control and manipulation of spin degrees of freedom in solid-state systems. It reviews the current status of spintronics, including recent advances and established results, with a primary emphasis on the fundamental physical principles underlying carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. The authors discuss spin decoherence mechanisms in metals and semiconductors in detail and apply various theories of spin injection and spin-polarized transport to hybrid structures relevant to spin-based devices and fundamental studies of material properties. Experimental work is reviewed, with a focus on projected applications where external electric and magnetic fields and quantum effects are harnessed to control and manipulate charge dynamics, creating new functionalities not feasible with conventional electronics. The article also covers the history and background of spin-polarized transport and magnetoresistive effects, as well as the techniques for generating spin polarization, such as optical orientation and electrical spin injection.