Abstract A contrast agent is often required to aid visualization of physiological structures. Recent advances in bionanomaterials have become pivotal in generating high-resolution, high-contrast images needed for accurate, precise, and noninvasive diagnostics. Bionanomaterials play major roles in imaging by delivering large imaging payloads, and yielding improved sensitivity, multiplexing capacity, and design modularity. Indeed, for several imaging modalities, bionanomaterials are now not simply ancillary contrast entities, but are instead the original and sole source of image signals that make possible the modality’s existence. In this chapter, we introduce the mechanisms of common imaging modalities, and the associated bionanomaterials that have been applied for enhancing imaging contrasts. A basic guideline for selecting bionanomaterials as imaging contrast agents for the different modalities is described.