This guide covers expert witness practice from selection through appeal. The opening parts set out the governing rules and the Supreme Court trilogy that established judicial gatekeeping, then analyze each element of the admissibility standard separately — qualification, helpfulness and fit, sufficiency of facts and data, reliability of principles and methods, and reliable application to the facts of the case. A dedicated part catalogues the recurring grounds of exclusion, from the analytical gap through methods created for litigation, inadequate differential analysis, extrapolation beyond the data, opinions outside the witness's field, and impermissible legal conclusions. Later parts cover disclosure obligations and the distinction between retained and non-retained experts, the protections for drafts and communications and their limits, deposition strategy, the mechanics of a motion to exclude, and the alternatives available when an opponent's expert is weak but not excludable. Separate parts address criminal cases and forensic disciplines under the Confrontation Clause, and appellate review of exclusion rulings. Tables, question sets, and worked examples appear throughout.