Beside the chemical purity, the structural purity is a concept that needs to be clarified. Indeed no clear definition can be encountered in literature although it has a dramatic influence on: physical characteristics, ageing, litigation about infringement. etc. A definition is proposed and discussed. Up to now no panacea has been found to enable the assessment of the structural purity. Therefore. in the case of polymorphism (the concept of structural purity also applies to solvates and amorphates), the best possible analyses rely oil a case by case basis. XRPD. IR, Raman, ss-NMR, DSC, M-DSC, are usually the most successfully tested methods. Some typical situations are presented and analysed. (c) 2006 Elsevier B.V. All rights reserved.