Neuronal MTs are structurally similar to those found in other eukaryotic cells [1]. The core structure is a polymer of 50-kDa tubulin subunits. Heterodimers of α- and β-tubulin align end to end to form protofilaments, 13 of which join laterally to form a hollow tube with an outer diameter of 25 nm (Fig. 8-2). Examples also exist of MTs with 12 and 14 protofilaments. The α- and β-tubulins are the best known members of a unique protein family, the members of which have significant sequence similarity [2]. There is approximately 40% sequence identity between α- and β-tubulins and even greater identity within the α and β gene subfamilies. Conservation of the primary sequence for tubulins is also high across species so that tubulins from yeast can readily co-assemble with tubulins from human brain. Tubulin dimers bind two molecules of GTP and exhibit GTPase activity that is closely linked to assembly and disassembly of MTs [1,3]. While many questions remain about tubulin and its interactions, the structure of the αβ-tubulin dimer has recently been derived from electron diffraction studies [3], providing a basis for dissection of the functional architecture of MTs.