A tremor is an involuntary, rhythmic shaking movement—but it is a symptom, not a diagnosis. Tremors may occur at rest, during movement, while holding a position, or when contracting muscles against resistance. Causes range from stress, caffeine, and medications to essential tremor, Parkinson’s disease, metabolic disorders, and other neurologic conditions. Understanding when and how the shaking occurs can provide an important clue to what is causing it.