Structure-based protein engineering has enabled the creation of designed molecular scaffolds that display a wide variety of sizes, shapes, symmetry, and subunit composition, ranging from small designed folds to large multi-protein assemblages. Such constructs offer the possibility to create protein-based nanoparticles for a wide variety of biotech and biomedical applications. One such family of designed proteins, termed circular tandem repeat proteins (‘cTRPs’) contain repeated protein sequences and structural motifs and form closed circular structures. The display of functional protein domains at regular intervals around their periphery can lead to enhanced properties as the result of increased avidity, combined with enhanced solution behavior and stability. I will describe the design, validation, characterization and functionalization of de novo designed cTRP proteins and provide examples of their application.