The quintessential Renaissance Man, Leonardo da Vinci was well aware of the fundamental importance of mathematics for architecture. This issue of the Nexus Network Journal examines Leonardo's knowledge of theoretical mathematics, explores how he used concepts of geometry in his designs for architectural projects, and reports on a real-life construction project using Leonardo's principles. Authors include Sylvie Duvernoy, Kim Williams, Rinus Roelofs, Biagio Di Carlo, Mark Reynolds, João Pedro Xavier, Vesna Petresin, Christopher Glass, and Jane Burry. To complete the issue Rachel Fletcher writes her Geometer's Angle column on "Dynamic Symmetry", Michael Ostwald reviews A Theory of General Ethics by Warwick Fox, Sarah Clough Edwards reviews Inigo Jones and the Classical Tradition by Christy Anderson, and Sylvie Duvernoy reviews Architecture and Mathematics in Ancient Egypt by Corinna Rossi.