The Hilbert problems where a list of 23 broad mathematical problems published by David Hilbert in 1900 which paved the way for some of the most influential works of the 20th century. In this talk I will give an overview of Hilbert's 21st problem, which asks whether or not we can find a linear differential equation on the complex plane with prescribed singular points and monodromy. First I will give an overview of the original problem and of the history of its proof. Afterwards, I will explain how generalizations of this problem led to the development of works by Deligne, Katz, Kashiwara, Mebkhout, etc on regular connections and the Riemann-Hilbert correspondence.