The packing of a protein's constituent atoms and the attendant constraints placed upon them form the basis of many attempts to understand and predict protein structure, stability, folding and even function. Although the significance of packing is yet to be fully comprehended, recent experimental and theoretical investigations have increased our understanding through the description of mutational effects on structure and stability, determination of the limits of packing constraints for both protein folding and structure prediction, and delineation of packing guidelines on the basis of observed cavities in the native protein folds. These advances and allowing protein modellers, engineers and designers to tackle their problems from a more rational perspective.