Here, the mesoscopic domain is defined to be between the size of biomacromolecules and that of a cell. It is in this domain that biomacromolecules often undergo pre-programmed assemblies and form complex machinery, and many interesting concepts start to emerge, such as material elasticity, mechanical strength, phase transition, spontaneous assembly, and compartmentalization. Traditionally, due to the lack of high-resolution tools to study these mesoscopic objects, many of their biological function and biophysical properties were not well understood. As we accumulate knowledge on these interesting biological objects in the coming decades, we envision an avenue towards applying synthetic biology and engineering principles to them that will provide innovative solutions to fundamental research, human health, and material science.