In this talk, I will first review the history of interior C^2 estimate for fully nonlinear equations. It turns out that very few equations were known to have such properties. 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 Monge-Ampere equations, Hessian equations and special Lagrangian equations. I will then discuss the main idea behind the proof. The new method we adopted to prove the interior C^2 estimate has independent interest and can be used in other settings.