Ab initio QMC methods, particularly diffusion Monte Carlo (DMC), are regarded as the most reliable practical approach for describing quantum many-body interactions (If one limits the scope to “only small molecular systems” or “simple models,” then highly reliable methods tailored by theorists can indeed be constructed in abundance. However, such methods are not easily applicable to realistic materials in practical materials science.). In this chapter, we explain why ab initio QMC methods are capable of achieving a higher level of descriptive accuracy, compared to other practical methods such as DFT or molecular orbital methods. We aim to deepen understanding of the relationship with other methods, as well as the differences in approach, thereby elucidating the features of the method itself.