Loneliness is a global public health concern. Many studies have linked it to high mortality, morbidity, and various psychological problems. Research now suggests that loneliness is a bi-dimensional construct made up of two related but distinct constructs; social loneliness (number of connections) and emotional loneliness (quality of connections). To inform intervention strategies and aid clinicians in tackling this growing ‘epidemic’, a systematic review of a widely used scale to measure emotional and social loneliness (DJGLS-6) was conducted. Two papers reviewed suggested the scale is reliable and valid. Next, data from a nationally representative sample of adults (N=1,839) was used to evaluate the factor structure of the DJGLS-6. The fit statistics for the one and two-factor CFA models were not acceptable. Modification indices indicated that adding a cross-factor loading to allow one item of the social loneliness factor, to load on to both loneliness factors improved the fit of the model significantly. The relationship between both constructs and a multitude of demographics was then tested using a series of regression analyses which further highlighted the distinction between the subtypes. Using both constructs as unique mediators, the relationship between traumatisation and traumatic disorders was also analysed. Emotional, not social, was found to significantly mediate that association. Finally, the possible co-occurrence of the loneliness subgroups and various clinical disorders was tested using LPA. Both subtypes co-occurred with the established disorders in a similar way. This thesis serves to provide evidence regarding the bi-dimensional nature of loneliness and advocate for researchers and health care providers to acknowledge this and provide evidence to support the argument that chronic emotional and social loneliness should be considered in future revisions of diagnostic manuals. Future work will focus on exploring other possible associations with specific disorders and proposing that loneliness is not just a social concept, but a threat to global mental and physical health and as such, requires a novel perspective of how it is conceptualised and treated.