I will discuss two examples of nanoscale self-assembly and the geometric principles that underpin them. The first is the formation of “nanosheets” from peptoids, structural isomers of peptides. Nanosheets are two molecules thick and many microns in planar extent, and I will describe the geometric principle that allows highly flexible peptoid polymers to form such a material. Second, I will discuss the self-assembly of small organic molecules into tilings on surfaces, and show how the properties of these tilings can be understood from the symmetry of their molecular building blocks. Knowing these geometric principles may allow us to make predictions for the self-assembly of a wide range of molecules.