This proposal will transform the area of organic crystal engineering by introducing a new level of ‘designability’ into functional molecular crystals. In the last 20 years, extended frameworks, and particularly metal-organic frameworks, have changed the perception of what is possible in terms of purposeful crystal engineering. This is because these frameworks comprise strong and directional extended bonding. By contrast, molecular crystals are not usually dominated by a single, directional motif. It remains highly challenging, therefore, to predict structure in molecular organic crystals, despite their enormous potential for synthetic diversity and function. If crystal structure is not predictable then ‘design’ of function is impossible. We will develop ‘robust organic tectons’—that is, organic molecules that assemble in a modular and predictable way without forming intermolecular coordination or covalent bonds.Our ambitious end goal, which goes beyond the state-of-the-art, is to predict physical properties for organic molecules a priori, based only on chemical formulae, thus guiding the synthetic programme. We will target solids with unprecedented properties—for example, chiral porous organic crystals that combine both shape selectivity and site-isolated, ‘orthogonal’ functionality, inspired by enzymes. To take a longer view, modular and computationally-led engineering of organic crystals could underpin future applications that are conceptual at present, such as molecular computing.The proposal comprises an integrated blend of chemical synthesis, supramolecular synthesis, characterization (e.g., PXRD), and computation (e.g., crystal structure prediction and molecular dynamics). Overall, we would summarize this as materials chemistry, but underpinned by physical chemistry and computation.