Additive manufacturing enables the exploration of process-structure-property relationships in ceramic and thermoplastic materials. The research presented in this dissertation is focused on tailoring function via structure, in terms of both geometry and composition. This methodology is applied to two areas: (a) advanced ceramics for dynamic environments and (b) single-piece prosthetic limbs 3D printed in thermoplastic materials.While advanced ceramics have many exceptional properties, their low damage tolerance remains a major challenge for real-world use, especially in dynamic environments. We propose that heterogeneous structuring will unlock extrinsic mechanisms that improve damage tolerance, which will enable the use of printed ceramic components for structural applications in the aerospace, defense, and energy sectors. Carbide specimens with composition gradients and layered structures are fabricated using a custom direct ink writing system. Sintering of multi-phase ceramics is found to generate residual stresses due to coefficient of thermal expansion mismatch. Printed specimens are characterized and mechanically tested in quasi-static and dynamic strain-rate regimes. Process-structure-property relationships are discussed with a focus on dynamic mechanical response. Recent advances in 3D scanning and computer-aided design enable the creation of ultra-personalized medical devices based on a patient’s ‘digital twin’ or computer-generated model from 3D scan data. Medical devices, such as prosthetic limbs and orthotic braces, can be designed around the patient’s digital twin to optimize fit and function, leading to improved comfort and medical outcomes. The presented research explores the fused-filament fabrication technique to create an affordable, single-piece transtibial (below-knee) prosthetic limb. The effect of printing parameters and material selection on mechanical strength are explored for several engineering-grade thermoplastic materials, with and without chopped fiber. Unfilled nylon is found to have an optimal balance of interlayer bonding strength and flexibility for prosthetic devices, while carbon-fiber filled nylon is shown to increase energy storage and release during the gait cycle. A patent-pending prosthesis is successfully developed and deployed in an underserved community of Ensenada, MX.