abstract: I will begin with some basic notions in 4-dimensional Riemannian geometry in order to define the concept of half-conformally-flat metrics (which are also known as self-dual or anti-self dual metrics), and a generalization of these known as Bach-flat metrics. These equations are elliptic in a suitable gauge, and I will discuss a basic regularity theorem. I will also discuss volume growth and Cheeger-Gromov convergence of such metrics, and discuss some explicit examples.