The crystal structure of one of the most well-known analgesics, acetylsalicylic acid, commonly referred to under its trade name aspirin, was first characterised by Wheatley in 1964. After the initial crystal structure was determined, it was widely discussed whether other crystal forms of aspirin may exist, but it was not until 2005 that Vishweshwar et al. reported the crystal structure of aspirin form II. The form II structure had been identified as an energy minimum by Ouvrard and Price 3 in a crystal-structure prediction exercise, but sub-optimal features of the experimental structure determination left doubts over the validity of the crystal structure and the existence of form II as a discrete polymorph. The origin of these difficulties was explained by Bond et al., who showed that aspirin forms I and II exist as intergrown single crystals, comprising domains of both structure types. Reproducible crystallisation of form II remained challenging, but it was shown in 2010 that domains of the second polymorph of aspirin are introduced reliably in the presence of aspirin anhydride during the crystallisation process. More polymorphs of aspirin have since been discovered. In this paper, we focus on the closely-related aspirin polymorphs, form I and form II.