The dependence of crystal structure assembly on molecular changes has been studied for mono-substituted acylanilides. A library of 255 crystal structures has been investigated. 217 structures were determined during this project and 38 originated from the CSD. A subset of 64 mono-substituted acetanilides was studied in detail for crystal structure similarities using the XPac algorithm and conformational preferences were assessed. Chapter 4 presents the crystal structures and their structural similarities, whilst Chapter 5 focuses on the theoretical conformational analysis. Three amide-amide hydrogen bond geometries and a molecular stack were frequently observed in the acetanilides. The assembly of these one-dimensional packing features governs in particular the crystal structures of the para substituted acetanilides. The amide-amide hydrogen bonded chains were very persistent in all the acylanilides. The amide substituent had a larger effect on the chain geometry than the phenyl substituent. In the crystal the molecular structures deviate from the expected planar conformation. Rotation about the amide to phenyl bond of up to 55 degrees in ortho-tertbutyl acetanilide is observed. Para-substituents affect the rotational barriers; whilst such correlation was not discernible for meta-substituents. Ortho substitution frequently produces intramolecular hydrogen bonding outweighing any other substituent effects. Although the experimental conformations deviate from the planar geometry they are still within the minimum energy well and energy penalties are small. These can be easily compensated for by intermolecular interactions in the crystal