Four-dimensional superconformal field theories (SCFTs) with 16 (or more) supercharges admit a protected subsector captured by a two dimensional chiral algebra (the holomorphic sector of a 2d CFT, called vertex operator algebra in the mathematics literature), allowing to obtain exact results in these theories. Surface defects, with sufficient supersymmetry, that intersect the chiral algebra at a point appear as point-like operators in the chiral algebra. We will introduce the four-dimensional SCFTs and their protected subsector, first without defects and then we describe how to introduce defects. We will discuss what four-dimensional observables in the presence of a defect are computable chiral algebra, and what the chiral algebra looks like in the presence of such defects.
