The synthesis of single crystal materials spanning macroscopic dimensions has enabled the fundamental understanding of structure-property relationships that have spurred revolutions in fields such as microelectronics, optics, and energy conversion. Block copolymers form ordered assemblies at length scales between that of molecular and colloidal crystals. Self-assembly occurs by confined phase separation, where two or more polymers, covalently bound to each other, fill their miscibility gap with ordered morphologies containing periodicities on the length scale of the polymer chain, roughly 10-100 nm. While the type of morphology and periodicity can be exquisitely controlled, only the simplest of self-assembled morphologies have been successfully prepared as "mesophase" single crystals with coherent periodic domains spanning macroscopic distances. This dissertation focuses on the complex ordered network structure known as the double gyroid (space group #230, Ia3d), which is prepared by the coassembly of a triblock terpolymer, poly(isoprene)-b-poly(styrene)-b-poly(dimethylaminoethylmethacrylate) (PS-b-PS-b-PDMAEMA, or simply ISA), blended with an oligomeric ceramic precursor, poly(methyl-vinyl-silazane) (PMVS). For the first time, this structure is assembled into single crystal domains that can be identified by eye and easily isolated for further study. These gyroid mesophase crystals span up to 14 mm2, more than three orders of magnitude larger than previously reported. These gyroid nanocomposites are characterized primarily with small angle x-ray scattering (SAXS) and electron microscopy. Importantly, the ISA/PMVS blends can be converted to ordered porous single crystal ceramic monoliths retaining the double gyroid morphology past 1100 ˚C, which can subsequently be used as templates for the infiltration of materials with specific functionality (e.g. superconductors). These mesophase single crystals are needed to realize angle-dependent and emergent properties predicted by theory/simulation such as negative refractive index. This dissertation begins with an introduction (Chapter 1) discussing classical nucleation theory and block copolymer thermodynamics. Chapter 2 documents the first preparation of the double gyroid morphology from ISA/PMVS blends and their transformation to a double gyroid mesoporous ceramic. Chapter 3 describes the preparation and characterization of double gyroid mesophase single crystals. Chapter 4 examines the behavior of the ISA/PMVS morphology diagram beyond the compositions previously studied. The conclusion (Chapter 5) suggests future experiments to expand upon findings reported in Chapters 2-4.