Yield-stress fluids are materials that transition from solid-like to fluid-like at a critical applied stress and are currently the most utilized rheological phenomenon. Yield-stress fluids have found use in drug delivery, food products, batteries, surface coatings, 3D printing materials, and many other applications. This rheological phenomenon can be achieved by a diverse range of microstructures including polymeric gels, colloidal glasses, and more. Rationally designing such rheologically complex materials requires the determination of the relationships between processing, structure, properties (rheology), and ultimately performance. This research is composed of distinct but interconnected experimental studies of these design relationships for yield-stress fluids. This work presents a paradigm for the design of rheologically-complex materials focused on the rheology-to-structure inverse problem for model yield-stress fluids which forms the basis for the subsequent studies that focus on a particular secondary property (extensibility), particular applications (direct-write 3D printing and performance magic), and appropriate processing to obtain a yield-stress fluid material. I generate a design space for material selection and design of extensible yield-stress fluids and introduce model materials with this important behavior. One model material is the subject of a case study on direct-write 3D printing; an emulsion with high extensibility is used to establish key property targets for direct-writable materials. Another study discusses the role of a yield stress and high extensibility in a resin used in performance magic, “Mystic Smoke”. The final study focuses on the connection between processing and rheology for a particular material, aqueous methylcellulose, and investigates the effects of dynamic conditions on the linear and nonlinear mechanical properties of gelling materials or yield-stress fluids.