The application of supramolecular concepts such as self-assembly to the solid state offers an approach to crystal design and crystal engineering, namely supramolecular synthesis of solids, that is based upon the design of infinite networks. Self-assembly of more than one molecular component, modular self-assembly, is particularly attractive since it can be accomplished in one-pot reactions with existing molecular components and allows for facile fine-tuning of structural and functional features. The challenges and opportunities that face crystal engineering are illustrated by concentrating on the superstructural diversity that has been exhibited in 2D network structures. Despite the observed superstructural diversity, which can manifest itself in the form of supramolecular isomerism, and the range of molecular components that have been utilized, these structures have in common an inherent ability to mimic clays by intercalation of guest molecules.