Steam-assisted gravity drainage (SAGD) is an effective technique to develop asphalt/heavy oil reservoirs. Although the concept of SAGD seems quite simple, there are many flaws in the theories established to understand this process. In fact, SAGD is a multi-physical process involving simultaneous heat and mass transfer. Specifically, it can be divided into four stages: stage of loop preheating by steam injection, vertical expansion stage of steam chamber, horizontal expansion stage of steam chamber and descend stage of steam chamber. And the recovery mechanism of SAGD includes steam viscosity reduction, gravity drainage, geomechanical effects, thermal effects, steam chamber coalescence and in-situ emulsification effects. Some of above mechanism are controversial, and the coupling effects among them may exist. In the future, more laboratory experiments and numerical simulations should be carried out for further exploration of such coupling mechanism, so that we can better understand, optimize and design the development process of SAGD. Therefore, the comprehensive effect of various mechanisms can be fully considered in actual application scenarios, so as to provide more reliable approaches of evaluation, prediction and design for SAGD process. Simultaneously, it is necessary to fully understand the recovery process and mechanisms of conventional SAGD, so that more reasonable variant forms of SAGD can be established.