The regularity of the reference domain in a boundary value problem plays a crucial role in determining the global regularity of the solution. While classical results assume smooth domains, namely of class $C^2$, many regularity results remain valid even when the domain lacks smoothness, provided it satisfies certain weaker geometric or analytic conditions. Nevertheless, the proof of such results often relies on a regularization procedure, where the reference domain is approximated by a sequence of smoother ones which preserve the key geometric and analytic features of the original one. The goal of this talk is to present various regularization techniques for different classes of non-smooth bounded domains of $\mathbb R^n$. We will discuss methods tailored to specific types of irregular boundaries, with examples including convex domains, sets of finite perimeter and domains whose boundaries exhibit Sobolev-type regularity.