Algebraic groups and $G$-complete reducibility: a geometric approach

The notion of a \emph{$G$-completely reducible} subgroup is important in the study of algebraic groups and their subgroup structure. It generalizes the usual idea of complete reducibility from representation theory: a subgroup $H$ of a general linear group $G={\mathrm{G}\mathrm{L}}_n(k)$ is $G$-completely reducible if and only if the inclusion map $i\text{\hspace{0.17em}⁣}:H\to {\mathrm{G}\mathrm{L}}_n(k)$ is a completely reducible representation of $H$. In these notes I give an introduction to the theory of complete reducibility and its applications, and explain an approach to the subject using geometric invariant theory.