In this talk, I will first review the history of interior C^2 estimate for fully nonlinear equations. Notably, very few equations were shown to admit such property. In the second part, I will discuss my recent work on interior C^2 estimate for Hessian quotient equations. Such equations have deep connections with the Monge-Ampere equation, Hessian equations and special Lagrangian equations. I will then discuss the main idea behind the proof and some future directions in this area.