Meningitis is an inflammatory syndrome involving the meninges that classically manifests with headache and nuchal rigidity and is diagnosed by cerebrospinal fluid examination. In contrast, encephalitis refers to inflammation of the brain parenchyma itself and often results in focal neurologic deficits or seizures. In this article, the authors review the differential diagnosis of meningitis and encephalitis, with an emphasis on infectious etiologies. The recommended practical clinical approach focuses on early high-yield diagnostic testing and empiric antimicrobial administration, given the high morbidity associated with these diseases and the time-sensitive nature of treatment initiation. If the initial workup does not yield a diagnosis, further etiology-specific testing based upon risk factors and clinical characteristics should be pursued. Effective treatment is available for many causes of meningitis and encephalitis, and when possible should address both the primary disease process as well as potential complications.