Precision Medicine in Neurodegenerative Disorders, Part Two, Volume 193  in the Handbook of Clinical Neurology deals with the "How" in the reconfiguration of our approach to slow accelerated brain aging. The book rethinks  animal models on which therapies are tested, outlines the progress and expected changes in biological subtyping efforts using lysosomal, endosomal, mitochondrial, immune dysregulation, and inflammatory mechanisms of disease pathophysiology, and the growing role of microbiome in shaping disease. The volume separates the potentially disease-modifying neurorescue and neurorestoration, (e.g., gene therapy and cell replacement therapy) from true precision "medicine"–matching biology with the mechanism of intervention of interest. Specific chapters are dedicated to the promise and challenges of extracellular vesicles for both diagnosis and treatment, the growing application of digital measures and other evaluations of clinical response, the nuts and bolts of novel adaptive clinical trial designs, and the regulatory changes needed to facilitate drug development for disease-modification purposes.