In this careful study of the origins and initial development of the catalogues of heretics that were to make such a long-lived and distinctive contribution to the idea of orthodoxy in the Early Church, and to the battle against all that supposedly threatened it, Geoffrey Smith adds to the growing interest in the construction and function of the idea of heresy as also of a Christian identity. The narrative that he produces runs from the antecedents before Justin through to its firm establishment by Irenaeus. However, within this comparatively brief framework Smith offers four major challenges to scholarly consensus: first, that despite the parallels often drawn between the catalogues and the doxographic tradition in contemporary philosophy, they owe more to the pseudepigraphic Pauline Epistles, canonical and non-canonical, in which the idea of succession and the call to guard the deposit of faith first emerges. Second, that Justin was not the author of the Syntagma against all heresies, but rather that it was already available in his circle and he successfully promoted it to a wider readership (Apol. 26.8); Smith subsequently follows the consensus that in Book i of Against heresies Irenaeus drew on the Syntagma, which had already been modified after it left Justin's hands. Third, that traces of other contemporary catalogues are to be found, for example in the lists of Jewish ‘heresies’ referred to by Hegesippus (Eusebius, HE iv.22) and by Justin (Dialogue 80.4), as well as in the Tripartite tractate (108–9, 112–13) and the Testimony of truth (55–9). Fourth, that Irenaeus applied the term ‘Gnostic hairesis’ or ‘school’ to his list of heretics drawn from the Syntagma in order to differentiate them from the universal Church, but that there is no evidence that they, or in particular the Valentinians, used the term ‘school’ of themselves – they saw themselves as part of the one Church. Smith uses this last argument to contradict those scholars who have understood Valentinus, and others, within a school framework; here, some might feel that Smith misunderstands recent modelling of early Christian diversity in Rome and Alexandria in terms of ‘schools’. Smith's style is a model of clarity, and his narrative embraces a number of informative excursions, for example the audience of Justin's Apology, the identity of the Jewish ‘heresies’ of Justin's Dialogue, or the ways in which books might be promoted and circulated among early Christian readers. He quotes and discusses carefully the pertinent texts and demonstrates a confident familiarity with much of the relevant recent literature. Thus, Guilt by association would serve as a stimulating introduction for those unfamiliar with the literature; at the same time, even if specialists in the field are not persuaded at every point, they will have to take very seriously both specific suggestions and his overall portrayal. Guilt by association is to be welcomed for its decisive invitation to others to engage with this, sometimes disturbing, tradition.