Infidelity violates a western norm that a range of interpersonal behaviours should remain exclusive in committed romantic relationships. Once exposed, the aftermath can be detrimental to all concerned. However, despite a cultural majority endorsing this belief and apprised of the potential consequences of its violations, infidelity or extradyadic relationships are widespread. Furthermore, individual differences in beliefs about what constitutes infidelity blur the boundaries of acceptable behaviour, making the concept of unfaithfulness difficult to fully describe. This variation in attitudes and behaviour, along with the consistent media attention infidelity attracts affirms the enigmatic nature of the behaviour. In response, an aim of the study was to clarify the construct of infidelity among a contemporary Australian sample. This was achieved in two ways. Firstly, the study examined beliefs and behaviours associated with unfaithfulness using qualitative (focus group) and largely quantitative (survey) data. Secondly, the study involved investigation of the association between infidelity and several individual, relationship and environmental variables. Differences in relationships and environmental conditions can be conceptualised within the theoretical framework of the investment model, while adult attachment theory and a lovestyles typology offer theoretical underpinnings to the study of individual differences. Specifically, the aim of this stage of the study was to examine how adult attachment styles (anxious, avoidant), lovestyles (eros, ludus, storge, mania, pragma, agape), relationship variables (satisfaction, investment, commitment), and an environmental variable (perceived alternatives) predicted infidelity. The sample comprised 243 women and 69 men between the ages of 18 and 60 years (M = 31.3 years, SD = 11.9) who were currently in a romantic relationship of at least one year or who had recently been in such a relationship. Participants completed measures pertaining to attachment, lovestyles and various aspects of relationship quality in addition to several measures of extradyadic behaviour. The study found that infidelity, as defined by respondents, was engaged in by 20% of individuals in their current relationships and by 42% of individuals in their previous relationships. Regarding the nature of infidelity, the current findings indicated that various sexual and emotional behaviours carried out with someone other than one's primary partner were considered unfaithful by the vast majority, while fantasy and flirting behaviours were generally seen as acceptable. It was noteworthy, however, that a substantial minority also viewed fantasy as unfaithful, underlining the inherent complexity of the construct. The hypotheses concerning the variables predicting infidelity were partially supported. Results suggested that individuals most likely to engage in extradyadic behaviour were those with an avoidant attachment style or a Ludus lovestyle, more perceived alternatives to their relationship, and most unexpectedly, higher levels of investment in their relationship. Conversely, those least likely to engage in these behaviours were those with an Eros Lovestyle and greater levels of commitment to their relationship. The study confirmed the prevalence of infidelity and emphasised the differential attitudes, behaviours and motivations associated with it. These differences are discussed in relation to the theories presented and an argument is made for research on infidelity to take a broader focus, one that includes the combined aspects of individuals, their relationships and the environment. Implications of these findings for individuals and couples and for the professionals who work with them are discussed, along with suggestions for future research.