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The anatomy of the alvarezsaurian tail has received relatively little attention in the paleontological literature, even though it shows a peculiar combination of anatomical characteristics that are unique among theropod dinosaurs. Nearly complete, informative tails are known from early-branching, intermediate, and late-branching taxa, allowing for robust inferences about their evolution. The alvarezsaurian tail is notable in being the longest among maniraptoran theropods, both in number of caudal vertebrae and proportional length. We examined the comparative anatomy and myology of the tail, and performed a cladistic analysis on a tail-character-based data matrix that provided a general framework for reconstructing alvarezsaurian tail evolution. Our results show that caudal vertebrae of alvarezsaurians have a combination of derived osteological features, intervertebral joint morphology, and inferred musculature …