The paper investigates the pretrial discovery process empirically. Using data from an interview survey of both attorneys in 369 federal civil cases, I identify factors that determine cases' discovery levels. The four main results suggest an overriding pattern: In the discovery process, plaintiffs behave differently from defendants. First, plaintiffs tend to conduct fundamentals discovery: The plaintiff chooses an amount of discovery by examining a case's underlying fundamentals, such as the amount at stake or the number of factual issues. The plaintiff does not increase her discovery in response to increased discovery by the defendant. Second, in contrast, the defendant's choice of discovery amount tends not to rely on fundamentals. Instead, the defendant chooses an amount that mirrors the plaintiff's amount, counter-punching in response to whatever discovery the plaintiff conducts. Third, the plaintiff and defendant respond to their adversary's apparently excessive discovery requests differently. If the plaintiff appears to have conducted excessive discovery, then the defendant retaliates: The defendant responds by conducting more discovery than she otherwise would have, almost exactly tit-for-tat. In contrast, the plaintiff responds to the defendant's apparent discovery aggression by retreating: The plaintiff conducts less discovery than otherwise. The litigants' behaviors of counter-punching, retaliating, and retreating demonstrate the interdependence of the amounts of discovery that each litigant seeks. Fourth, the nature of an attorney's fee arrangement influences the attorney's discovery behavior substantially. A litigant's discovery behavior also responds to characteristics of both the litigant and the adversary.