Due to its rising prevalence and associations with physical and mental health problems, loneliness is a public health concern (Office of National Statistics, 2021). This thesis aimed to explore factors involved in the aetiology of loneliness. Part one of this portfolio presents a systematic review examining how attachment styles relate to loneliness in adults. Findings suggest that individuals with insecure attachment styles, both anxious and avoidant types, are more likely to report loneliness. Part two of this portfolio presents an empirical paper that investigates whether adverse childhood experiences (ACEs) predict trajectories of loneliness over time, distinguishing between transient loneliness (episodic loneliness that comes and goes) and chronic loneliness (loneliness that is pervasive and enduring). Findings demonstrate that experiencing childhood adversity increases the likelihood of following a chronic loneliness trajectory but does not impact the likelihood of experiencing transient loneliness. This paper therefore sheds light on our understanding of chronic loneliness, which is important as chronic loneliness is associated with poorer health and wellbeing outcomes relative to transient loneliness. Together, both papers highlight that the way in which people appraise social relationships, and their past experiences with social relationships, are important predictors of loneliness which persists into adulthood. Clinically, this suggests that simply increasing the quantity of social contact will not be sufficient in tackling loneliness as this does not alter the way in which social relationships are appraised. This has important implications when we consider that social prescribing has been utilised as a primary means to tackle loneliness.