Our bodies make molecules when they break down food and when they are stressed. These molecules are called radicals. Free radicals are important for our bodies to work properly like when we're fighting off sickness and our cells are talking to each other. When we have too many free radicals it can cause problems. Our brain cells are very sensitive to damage from radicals. This is because brain cells use a lot of oxygen they have a lot of fat they do not have things to protect them from damage and they do not fix themselves very well. Free radicals can hurt our brain cells. Cause oxidative stress. Our brain cells are very important. When free radicals hurt them it can cause big problems. Free radicals are a problem, for our brain cells. Oxidative stress causes a lot of problems like lipid peroxidation, protein oxidation, mitochondrial malfunction, DNA damage, compromised synaptic transmission and neuronal apoptosis. Oxidative stress is the reason for neurodegenerative diseases such as Alzheimers disease, Parkinsons disease, amyotrophic lateral sclerosis and Huntingtons disease. These diseases happen because of the changes that oxidative stress causes. This review talks about the production and metabolism of radicals the molecular processes that cause neuronal damage the endogenous antioxidant defense systems and the connection between oxidative stress and neurodegeneration from a thorough biochemical standpoint focusing on oxidative stress and neurodegeneration and how oxidative stress affects the body. The review covers stress and its effects on the body including neurodegeneration to understand how oxidative stress leads to neurodegenerative diseases, like Alzheimers disease, Parkinsons disease, amyotrophic lateral sclerosis and Huntingtons disease. New ways to reduce damage and keep neuronal function working are also talked about along with new ideas, for using antioxidants to treat people. If we understand the pathways that connect free radicals to neuronal deterioration better this can help us find more effective ways to prevent and treat neurological illnesses like these neurological illnesses. The more we know about the pathways that connect free radicals to neuronal deterioration the more we can do to stop neurological illnesses from happening in the first place and the more we can do to treat neurological illnesses when they do happen.