Spin transfer torques (STTs) are forces that arise from the transfer of spin angular momentum between spin-polarized currents and magnetic materials. This article introduces the physics of STTs in magnetic devices, providing an overview of the mechanisms, theoretical and experimental progress, and their potential applications. It serves as an introduction to a series of articles on STTs published in the Journal of Magnetism and Magnetic Materials. The goal is to provide a basic background and context for the subsequent articles, which focus on specific aspects of STT physics and highlight unanswered questions for future research. The article begins by explaining that while electrons in non-ferromagnetic materials are spin-randomized, those in ferromagnetic materials can become spin-polarized, influencing device behavior. Spin transfer torques occur when the spin angular momentum of flowing electrons interacts with the magnetization of ferromagnetic elements, leading to changes in the magnetization direction. This effect is central to the study of STTs and has significant implications for magnetic memory and spintronic devices. The article reviews the history of STT research, starting with Berger's prediction in the 1970s and 1980s, followed by experimental observations of domain wall motion in thin ferromagnetic films. Advances in nanofabrication have enabled the study of STTs in smaller devices, leading to a surge in research since the early 2000s. The article also discusses the discovery of giant magnetoresistance (GMR) and its significance in magnetic tunnel junctions, which have become important for spintronic applications. The text explains the basics of ferromagnetism, including the role of exchange interactions and hybridization in determining magnetic properties. It introduces simplified models like the Stoner model and the s-d model to describe spin-polarized currents and their interactions with magnetic materials. The article also covers micromagnetic theory, which is essential for understanding the dynamics of magnetic domains and domain walls. The discussion of STTs includes the concept of spin current and how it can induce torques on magnetic materials. The article explains the mechanisms by which spin-polarized currents interact with magnetic layers, leading to changes in magnetization. It also addresses the dynamics of magnetic systems in the absence of STTs, using the Landau-Lifshitz and Gilbert equations to describe magnetization precession and damping. The article highlights the importance of STTs in magnetic memory devices, such as racetrack memory, and discusses the potential for STTs in high-frequency applications like microwave sources and detectors. It also touches on the challenges and opportunities in applying STT principles to real-world devices, emphasizing the need for further research and development. The text concludes with a review of relevant literature and resources for further study on spin transfer torques.Spin transfer torques (STTs) are forces that arise from the transfer of spin angular momentum between spin-polarized currents and magnetic materials. This article introduces the physics of STTs in magnetic devices, providing an overview of the mechanisms, theoretical and experimental progress, and their potential applications. It serves as an introduction to a series of articles on STTs published in the Journal of Magnetism and Magnetic Materials. The goal is to provide a basic background and context for the subsequent articles, which focus on specific aspects of STT physics and highlight unanswered questions for future research. The article begins by explaining that while electrons in non-ferromagnetic materials are spin-randomized, those in ferromagnetic materials can become spin-polarized, influencing device behavior. Spin transfer torques occur when the spin angular momentum of flowing electrons interacts with the magnetization of ferromagnetic elements, leading to changes in the magnetization direction. This effect is central to the study of STTs and has significant implications for magnetic memory and spintronic devices. The article reviews the history of STT research, starting with Berger's prediction in the 1970s and 1980s, followed by experimental observations of domain wall motion in thin ferromagnetic films. Advances in nanofabrication have enabled the study of STTs in smaller devices, leading to a surge in research since the early 2000s. The article also discusses the discovery of giant magnetoresistance (GMR) and its significance in magnetic tunnel junctions, which have become important for spintronic applications. The text explains the basics of ferromagnetism, including the role of exchange interactions and hybridization in determining magnetic properties. It introduces simplified models like the Stoner model and the s-d model to describe spin-polarized currents and their interactions with magnetic materials. The article also covers micromagnetic theory, which is essential for understanding the dynamics of magnetic domains and domain walls. The discussion of STTs includes the concept of spin current and how it can induce torques on magnetic materials. The article explains the mechanisms by which spin-polarized currents interact with magnetic layers, leading to changes in magnetization. It also addresses the dynamics of magnetic systems in the absence of STTs, using the Landau-Lifshitz and Gilbert equations to describe magnetization precession and damping. The article highlights the importance of STTs in magnetic memory devices, such as racetrack memory, and discusses the potential for STTs in high-frequency applications like microwave sources and detectors. It also touches on the challenges and opportunities in applying STT principles to real-world devices, emphasizing the need for further research and development. The text concludes with a review of relevant literature and resources for further study on spin transfer torques.