This is a Part III essay aiming to discuss the contruction of quantum error-correcting codes through the use of the theory of algebraic function fields, which produces codes with asymptotically good parameters. This exposition emphasises constructibility and several applications of the main theorem (theorem 6.2) are given. Prerequisites are the first 12 lectures of the Part III course Quantum Information Theory [16] or chapter 7 of Preskill’s Lecture Notes [15], and the first two chapters of Stichtenoth’s Algebraic Function Fields and Codes [19].