Although the discussion is general, this book focuses on the problem of macroscopic quantum phenomena using systems of spintronics. The spintronics considered are ferromagnetic and antiferromagnetic spintronics. To represent the macroscopic quantum phenomena in spintronics, transitions from one state to another of the magnetization of ferromagnetic spintronics are considered, and of the Néel vector of antiferromagnetic spintronics. The authors have studied transitions from a metastable state to a more stable one, as well as quantum coherence between two degenerate stable states. Quantum and classical rates of transitions are presented as functions of temperature, magnetic field and the spin-polarized current flowing through the spintronics. With this method, one can immediately observe the effect of the spin-polarized current on the transitions of the magnetization and the Néel vector when comparing the results to those of the earlier ones on magnetic systems that did not have spin-polarized current. Specifically, while dissipations in magnetic system are intrinsic, the book shows how the total dissipation in spintronics can be controlled and eliminated by varying the spin-polarized current appropriately that depends on the temperature. The study of transitions from a metastable state to a more stable one in ferromagnetic spintronics shows that the rate of transitions of the magnetization at low temperatures is low and vanishes at zero temperature, so that the magnetization is relatively more stable than that in ferromagnetic materials without existence of spin-polarized currents. In the case of antiferromagnetic spintronics, the behavior and characteristics of transitions of the Néel vector is in contrast to those of ferromagnetic spintronics, where the low-temperature rate of transitions in antiferromagnetic spintronics varies exponentially small in temperature, and is finite and non-vanishing at zero temperature. In addition to the theoretical aspects, the book also discusses experimental and technological aspects that one may obtain. Measurements of the rate of transitions can be used to provide an independent method to determine certain parameters being involved, such as the anisotropy parameter Kc of tetragonal crystals, which is an important parameter but usually difficult to obtain. Eliminating dissipation in ferromagnetic and antiferromagnetic spintronics would be desired so as not to have unnecessary loss of energy. Low rate of transitions corresponds to the initial state that is relatively stable. Technologically, the stability of the states of the magnetization and Néel vector in spintronics are important, for example, for memory storage.