Molecules to crystals, crystals to molecules ... and back again?

Self-assembly of more than one molecular component, modular self-assembly, is a particularly attractive approach to synthesizing new crystalline compounds, because it can be accomplished in one-pot reactions with existing molecular components and it allows for facile fine-tuning of structural and functional features. The challenges and opportunities that face crystal engineering are illustrated by concentrating on the structural diversity that has been exhibited in two classes of compounds that exemplify these opportunities: metal-organic nets and molecular co-crystals.