2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/16507I survey the kinds of critical behavior believed to be exhibited in two-dimensional disordered systems. I review the different replica sigma models used to describe the low-energy physics, and discuss how critical points appear because of WZW and theta terms.19 pages; Lecture at the NATO Advanced Study Institute/EC Summer School on New Theoretical Approaches to Strongly Correlated Systems, Newton Institute, Cambridge, UK, April 10-20, 2000Strongly Correlated ElectronsDisordered Systems and Neural Networkstext