This thesis first focuses on reviewing the literature in the field of persecutory and grandiose delusions and the role of self-esteem in their development and maintenance. An empirical study exploring first person accounts of grandiose beliefs is then considered. A systematic review of the literature was conducted to elucidate the role of self-esteem in persecutory and grandiose delusions. Electronic databases were searched and thirty four studies were included. The review yielded largely mixed results. A number of higher quality studies indicated that persecutory delusions are associated with low self-esteem and that they are predicted by fluctuations in self-esteem. There was some evidence showing that grandiose delusions are associated with higher self-esteem. Studies investigating grandiose delusions are scarce, suggesting a need for further high quality research in this area. An empirical study was conducted to explore the lived experience of individuals with grandiose beliefs, with the purpose of developing a theory of grandiose beliefs. Seven individuals were interviewed using a Semi-Structured Interview Schedule. A Grounded Theory method was used. The findings demonstrated a number of shared processes: Expanding Sense of Self, Higher Consciousness, Search for Healing, Re-gaining Control and Element of Truth and Validation. The developed theory suggested that multiple pathways could lead to the onset of grandiose beliefs, including a pathway leading from the experience of paranoid to grandiose beliefs. The implications of the developed framework of grandiose beliefs for future research and clinical practice are considered.