Intel declared 2016 as the end of Moore's prediction. Researchers and academicians are exploring other alternatives to fulfill the latency between the processor and memory system. A universal memory is required that can be used at the various levels of memory hierarchy. STT-MRAM has shown the promising features to be used at various levels of memory hierarchy. In this chapter, we discussed the GMR, TMR, and STT as the basic phenomena required for STT-MRAM reading and writing. Conversion of charge current to spin-polarized current is explained with the help of Bloch states of different symmetries. I-MTJ and P-MTJ are explained using key performance parameters such as thermal stability and critical current. Working of STT-MRAM bit cell is discussed using NMOS transistor as an access device. Framework for low power hybrid MTJ/CMOS circuits is explained using PCSA, CMOS logic tree, and nonvolatile input store in terms of relative magnetization state of MTJs. STT-MRAM faces the challenges of high write energy, reliability, and read disturbance due to common read and write path. To mitigate these issues, SOT-based device and fast-switching mechanism VCMA has been suggested. Finally, based on the performance of STT-MRAM, it can be projected that low-power operations can be achieved using STT-MRAM as a working memory. Further, high-speed and low-power operations can be attained with hybrid MTJ/CMOS nonvolatile core circuits. The recent developments in the spintronics field have opened the door for energy-saving and high-performance electronics from device level to circuit level.