Individuals learn to inﬂuence and manipulate others to function as part of society. Machiavellianism captures one’s willingness to orchestrate the behaviour of others against their interests, rights, and well-being. Research focuses primarily on a single Machiavellianism dimension. This thesis, however, contends that Machiavellianism comprises two correlated dimensions: a views dimension that captures one's cynical and distrusting view of humanity and the world, and a tactics dimension that captures one’s willingness to endorse exploitative and amoral behaviours when deemed advantageous. This thesis aimed to develop a stronger understanding of each dimension, and this required developing stronger psychometric instruments. The secondary aim was to test the presupposition of no psychopathological cost to Machiavellianism. After an initial foray into Machiavelli and Machiavellianism in the ﬁrst two chapters, Chapter 3 identiﬁes a robust Machiavellianism factor-structure and how each dimension relates to psychopathological domains in 1478 US and 218 Australian participants. Conﬁrmatory factor analysis demonstrated that Machiavellianism comprises two robust dimensions which could be reliably captured through a 10-item subset of the Mach-IV scale, named Two-Dimensional Mach-IV (TDM-IV). Further, Machiavellian views associated with all major psychological domains, while Machiavellian tactics related only to the externalising and thought dysfunction domains. Machiavellianism is two-dimensional, with each dimension having distinctive psychopathological implications. The study in Chapter 4 investigates whether these two dimensions are universal, or merely measurement artefacts within Study 1. If universal, this research further aimed to develop a nomological network to better understand the nature of each dimension. International collaborators shared 15 datasets, which comprisedover 17,000 participants. The two-factor structure was reproducible and structurally equivalent across cultures, languages, types of respondent, response category length, age, and gender. Further, each dimension was situated within a diﬀerent constellation of broad personality traits, developmental pathways, emotionality, and behaviour. Therefore, the two dimensions appear to be core aspects of Machiavellianism and need to be independently captured in future research. The TDM-IV derives from the Mach-IV, inheriting many of its psychometric concerns that reduce the accuracy of its inferences, such as confusing item wording and not accounting for acquiesces appropriately. To overcome these weaknesses, Chapter 5 presents the development and validation of the Two-Dimensional Machiavellianism Scale (TDMS). The TDMS had excellent psychometric properties in six independent samples involving over 3800 participants, based on conﬁrmatory factor analysis, longitudinal structural equation modelling, and item response theory. The scale provided invariant measurement across all samples and a test-retest sample, was internally consistent, and provided most information in the low to high average range. This study demonstrates conﬁrmatory and discriminatory validity with existing measures of Machiavellianism, broader personality taxonomies, socio-political attitudes, psychopathy, narcissism, and morality vignettes. Finally, Chapter 6 explicates this two-dimensional Machiavellianism construct and discusses key areas for future investigation, including latent proﬁles, longitudinal modelling of each dimension’s development, and cross-cultural equivalence. Together, this research demonstrates that: a) Machiavellianism comprises two distinct dimensions, b) the TDMS, as a psychometrically robust measure of Machiavellianism, should replace current measures of Machiavellianism, and c) the presupposition of psychopathological immunity among Machiavellians is false.