Free radicals are molecules, ions, or atoms with unpaired electrons in their outermost shell of electrons. Although it is a relatively recent discovery, the occurrence of free radicals in biological processes is now widely accepted. Free radicals are involved in the pathogenesis of many diseases, such as diabetes, cardiovascular and pulmonary diseases and also in inflammation. Free radicals are also involved in important physiological processes, such as ageing. Free radicals are constantly formed in the human body, but they can become toxic when generated in excess or in the presence of a deficiency in the naturally occurring antioxidant defences. In general, the body has adequate antioxidant defences to cope with the production of free radicals under physiological conditions. This book reviews the updated knowledge on free radicals and its possible mechanisms in neurological disorders and in some oxidative stress processes, such as in intractable epilepsy and febrile seizures. Some approaches useful to elucidate the effects of free radical exposure on the overall protein structure are also discussed. Additionally, the potential risk factors and major cellular events contributing to the generation of reactive oxygen species and retinal pigment epithelium (EPI)/photoreceptor cell damage, and their roles in the pathogenesis of AMD are examined. Furthermore, thyroid cells are a unique model to study free radical processes involving reactive oxygen and iodine species. The role of oxidative stress in impairment in the thyroid function is explored. Other chapters in this book explore free radicals and its role in the in vivo disease control of malaria, as well as the monitoring of free radical in ultrasonic field in vitro and in vivo, its role in laboratory medicine, the relationship between rabies and free radicals, the correlation between exercise and free radical production, and the role of free radicals in several homeostatic processes through methods that can be used to detect free radicals either directly or indirectly (also known as fingerprinting methods).