Microtubule nucleation is controlled by the γ-tubulin ring complex (γTuRC) and related γ-tubulin complexes, which provide spatial and temporal control over microtubule growth. Recent structural studies have clarified the mechanism of γTuRC-based nucleation, confirming that γTuRC functions as a microtubule template. Crystallographic analysis of the first non-γ-tubulin γTuRC component, GCP4, has improved understanding of the relationships among all γTuRC proteins, leading to a refined model of their organization and function. The structures also suggest an unexpected mechanism for regulating γTuRC activity via conformational modulation of the complex component GCP3. New experiments on γTuRC localization suggest a direct link between attachment at specific cellular sites and activation. The microtubule cytoskeleton is crucial for the spatial and temporal organization of eukaryotic cells, playing a central role in functions such as intracellular transport, organelle positioning, motility, signaling, and cell division. Microtubules are highly dynamic polymers that switch between cycles of growth and depolymerization. Cells have evolved various ways to manipulate microtubule dynamics to achieve precise control of the microtubule cytoskeleton. While many mechanisms regulate microtubule dynamics, the cell controls microtubule assembly and catastrophe rates, as well as the timing and location of nucleation events. Microtubules are hollow tubes assembled from αβ-tubulin heterodimers in a GTP-dependent manner. The tubulin subunits form two types of filament contacts: longitudinal contacts run the length of the microtubule forming protofilaments, and lateral contacts between protofilaments form the circumference of the microtubule. Microtubule geometry is not fixed, and the more flexible lateral contacts can accommodate between 11 and 16 protofilaments, yielding microtubules of different diameters when assembled in vitro. In vivo, almost all microtubules have thirteen protofilaments, suggesting that one level of cellular control involves defining a unique microtubule geometry. Thirteen-fold symmetry is likely preferred because it is the only geometry in which protofilaments run straight along the microtubule length, allowing processively tracking motor proteins to always remain on the same face of the structure. Another key difference between microtubule assembly in vivo and in vitro is how new microtubules are initiated. In vitro, microtubule growth must proceed through small, early assembly intermediates, in which disassembly is energetically favored over assembly to result in slow initial growth. After a sufficiently large oligomer has been achieved, microtubule growth becomes energetically favorable and the addition of tubulin heterodimers proceeds rapidly. Significantly, rather than relying on the spontaneous initiation of new microtubules, cells have evolved specialized nucleMicrotubule nucleation is controlled by the γ-tubulin ring complex (γTuRC) and related γ-tubulin complexes, which provide spatial and temporal control over microtubule growth. Recent structural studies have clarified the mechanism of γTuRC-based nucleation, confirming that γTuRC functions as a microtubule template. Crystallographic analysis of the first non-γ-tubulin γTuRC component, GCP4, has improved understanding of the relationships among all γTuRC proteins, leading to a refined model of their organization and function. The structures also suggest an unexpected mechanism for regulating γTuRC activity via conformational modulation of the complex component GCP3. New experiments on γTuRC localization suggest a direct link between attachment at specific cellular sites and activation. The microtubule cytoskeleton is crucial for the spatial and temporal organization of eukaryotic cells, playing a central role in functions such as intracellular transport, organelle positioning, motility, signaling, and cell division. Microtubules are highly dynamic polymers that switch between cycles of growth and depolymerization. Cells have evolved various ways to manipulate microtubule dynamics to achieve precise control of the microtubule cytoskeleton. While many mechanisms regulate microtubule dynamics, the cell controls microtubule assembly and catastrophe rates, as well as the timing and location of nucleation events. Microtubules are hollow tubes assembled from αβ-tubulin heterodimers in a GTP-dependent manner. The tubulin subunits form two types of filament contacts: longitudinal contacts run the length of the microtubule forming protofilaments, and lateral contacts between protofilaments form the circumference of the microtubule. Microtubule geometry is not fixed, and the more flexible lateral contacts can accommodate between 11 and 16 protofilaments, yielding microtubules of different diameters when assembled in vitro. In vivo, almost all microtubules have thirteen protofilaments, suggesting that one level of cellular control involves defining a unique microtubule geometry. Thirteen-fold symmetry is likely preferred because it is the only geometry in which protofilaments run straight along the microtubule length, allowing processively tracking motor proteins to always remain on the same face of the structure. Another key difference between microtubule assembly in vivo and in vitro is how new microtubules are initiated. In vitro, microtubule growth must proceed through small, early assembly intermediates, in which disassembly is energetically favored over assembly to result in slow initial growth. After a sufficiently large oligomer has been achieved, microtubule growth becomes energetically favorable and the addition of tubulin heterodimers proceeds rapidly. Significantly, rather than relying on the spontaneous initiation of new microtubules, cells have evolved specialized nucle