A spin-transfer torque switched magnetic tunnel junction (MTJ) is a memory element in modern magnetic random access memory (MRAM), a CMOS-integrated technology that is being developed by semiconductor manufacturers. One electrode of the MTJ acts as a magnetically bi-stable “free-layer” (FL), determining the bit-state of a memory cell. Spin-transfer torque (or spin-torque, or STT) switches the magnetic state of the FL with respect to a reference-layer (RL) in the MTJ. Here I review our current understanding of the MTJ device physics governing spin-transfer-torque driven switching. Starting with some simple concepts based on macrospin assumptions, I will then review recent experimental results, and compare observations with macrospin expectation for a more realistic, observation-based quantitative description, beyond macrospin. I will also describe some unresolved scientific challenges that could further improve the MTJ performance for memory technology, and give a brief outlook.