The large-charge expansion is a powerful approach for analytically accessing strongly-coupled models at or near criticality. We focus on a sector of fixed and large global charge where it is possible to express observables as an expansion in inverse powers of the charge. I will first introduce the approach using the simplest relativistic case, namely the O(2) model. Then I will discuss how to apply the large-charge expansion to non-relativistic models which is relevant for the study of ultracold Fermi gases and nuclear physics.