Nitric oxide (nitrogen monoxide; NO) is a simple molecule with diverse biological functions. NO and related reactive nitrogen oxide species (RNOS) mediate intricate physiological and pathophysiological effects in the central nervous system. Depending on environmental conditions, NO and RNOS can initiate and mediate neuroprotection or neurotoxicity either exclusively or synergistically with other effectors. The focus of this review is limited to the neuroprotectant/neurotoxic role of NO in Acquired Immune Deficiency Syndrome (AIDS) Dementia Complex (aka HIV—Associated Dementia; HAD) Amyotrophic Lateral Sclerosis (aka Lou Gehrig's Disease), Alzheimer's Disease, Huntington's Disease, Multiple Sclerosis and Parkinson's Disease. This review will shed light on the question: "How important is NO in neurodegenerative diseases?"