Manipulating the magnetization direction of a magnetic 346 layer with-out the use of an external magnetic field represents an outstanding opportunity and a challenge for spintronic applications [1]. In the past twenty years, vast experimental data and many detailed theoretical results have been produced. In this chapter, we aim to provide a coherent theoretical description of this topic for various phenomena involving the interplay between spin transport and magnetization dynamics. There are already a few excellent review papers [2–6] on this same subject, and we refer to some of the details in these articles. In this chapter, we address the theoretical aspects of current-induced -magnetization control using the so-called the spin torque effect (ST) from both the transport and magnetization dynamics points of view. Although numerous models have been developed, discussing them all is far beyond the scope of this chapter and we rather refer the reader to specific literature. After 347a brief overview of ST studies in Section 8.1.1, transport characteristics in a number of magnetic systems such as spin valves, magnetic tunnel junctions, and magnetic domain walls are described in Section 8.1.2. At the end of the section, several unconventional spin torques will be briefly discussed. In Section 8.1.3, the ST-driven magnetization dynamics for these magnetic systems is studied. Finally, perspectives and conclusions are given in Section 8.1.4.