Acknowledgements As is extremely common for large pieces of academic work, this thesis would not exist if it were not for many people other than myself. Also, this thesis was completed in part whilst receiving an STFC PhD studentship. I would like to start by thanking Anupam Mazumdar for several helpful discussions regarding thermalization and thermal interaction rates. I have had many helpful and insightful conversations with my academic peers and friends Phil Stephens, Frankie Doddato, Ernest Pukartas, Lingfei Wang, Mindaugas Karciauskas and Jacques Wagstaff. I would like to thank Jessica Brooks. The majority of the work on the new Thermal Inflation model that is introduced in this thesis was done in collaboration with David Lyth. A spe- cial mention needs to be given to David, however, as he was not merely an academic collaborator, but for a large part was also my acting supervisor. I have had many fruitful discussions on Particle Cosmology with David. I feel privileged to have worked with such a pillar of modern Cosmology. I would like to thank my very dear friends and family Linda Rumsey, Rob Bishop, Paul Evans, Matt Eve, Guy Rusha, Kelly Hodder, Tristan Reeves, Chelle Nevill and Alise Kirtley for the support and encouragement that you i