The Active Optics System will correct system aberrations introduced by gravity and temperature gradients in near real-time. This session will focus on the wavefront estimation component, where the wavefront is estimated from donut images on the corner wavefront sensors. We will discuss recent progress developing the curvature wavefront sensing technique and hear about alternative approaches based on optimization and machine learning. We may also have the opportunity to discuss the lessons learned processing donuts from the Dark Energy Camera and Hyper Suprime-Cam, or have a software tutorial.