Neuronal dysfunction and degeneration are central events of a number of major diseases with significant unmet need. Neuronal dysfunction may not necessarily be the result of cell death, but may also be due to synaptic damage leading to impaired neuronal cell signaling or long-term potentiation. Once degeneration occurs, it is unclear whether axonal or synaptic loss comes first or whether this precedes neuronal cell death. In this review we summarize the pathophysiology of four major neurodegenerative diseases; Alzheimer's disease, Parkinson's disease, multiple sclerosis and amyotrophic lateral sclerosis (Lou Gehrig's disease) For each of these diseases, we describe how biochemical biomarkers are currently understood in relation to the pathophysiology and in terms of neuronal biology, and we discuss the clinical and diagnostic utility of these potential tools, which are at present limited. We discuss how markers may be used to drive drug development and clinical practice.