ABSTRACT:
I will describe several recently posed conjectures that relate BPS states in four-dimensional N=2 quantum field theories to representation theory of non-unitary chiral algebras. These conjectures construct wall-crossing invariant generating functions of refined BPS indices which surprisingly are equal to characters of chiral algebras. These characters frequently enjoy nice modular properties. I will also discuss extensions to BPS states in the presence of defects which further enrich the correspondence.