Across the last several decades, the thin ideal (i.e., a female physique characterised by slenderness and low body fat) has featured as the predominant beauty ideal promoted to women living in Western societies. Unfortunately, the widespread Western cultural endorsement of thinness as a desirable and attractive physical attribute is considered a key contributor to the high rates of body image and eating concerns reported by women living in these societies (Grogan, 2021; Swami, 2015). Studies have shown that women are most at risk when they have internalised these cultural messages and adopted the thin ideal as a personal beauty standard to strive for. Indeed, thin ideal internalisation has been identified as an important risk factor in women's body dissatisfaction and disordered eating (Barakat et al., 2023; Paterna et al., 2021). Traditionally, thin ideal internalisation has been defined as the "extent to which an individual buys into socially defined ideals of attractiveness" (Thompson & Stice, 2001, p. 181). However, recent research has broadened the operationalisation of beauty ideal internalisation to include a range of cognitive, affective, and behavioural components (Uhlmann et al., 2020). Although this revised operationalisation distinguishes between multiple components of beauty ideal internalisation, this definition has not yet been applied to the thin ideal. Consequently, available measures of thin ideal internalisation are brief and unidimensional. The multidimensional operationalisation and measurement of thin ideal internalisation offers potential to improve theoretical and clinical understanding of this important construct in women's body image and eating. Therefore, the primary aim of this thesis was to develop and psychometrically validate a new measure of thin ideal internalisation that was designed to comprehensively assess the cognitive, affective, and behavioural components of thin ideal internalisation in women. [...]