Terrorists, drug traffickers, mafia members, and corrupt corporate executives have one thing in common: most are conspirators subject to federal prosecution. Federal conspiracy laws rest on the belief that criminal schemes are equally or more reprehensible than are the substantive offenses to which they are devoted. The Supreme Court has explained that a "collective criminal agreement—[a] partnership in crime—presents a greater potential threat to the public than individual delicts. Concerted action both increases the likelihood that the criminal object will be successfully attained and decreases the probability that the individuals involved will depart from their path of criminality." Moreover, observed the Court, "[g]roup association for criminal purposes often, if not normally, makes possible the attainment of ends more complex than those which one criminal could accomplish. Nor is the danger of a conspiratorial group limited to the particular end toward which it has embarked." Finally, "[c]ombination in crime makes more likely the commission of crimes unrelated to the original purpose for which the group was formed." In sum, "the danger which a conspiracy generates is not confined to the substantive offense which is the immediate aim of the enterprise." Congress and the courts have fashioned federal conspiracy law accordingly. The United States Code contains dozens of criminal conspiracy statutes. One, 18 U.S.C. 371, outlaws conspiracy to commit any other federal crime. The others outlaw conspiracy to commit some specific form of misconduct, ranging from civil rights violations to drug trafficking. Conspiracy is a separate offense under most of these statutes, regardless of whether conspiracy accomplishes its objective. The various conspiracy statutes, however, differ in several other respects. Section 371 and a few others require at least one conspirator to take some affirmative step in furtherance of the scheme. Most have no such explicit overt act requirement. Section 371 has two prongs. One outlaws conspiracy to commit a federal offense; a second, conspiracy to defraud the United States. Section 371 conspiracy to commit a federal crime requires that the underlying misconduct be a federal crime. Section 371 conspiracy to defraud the United States and a few others have no such prerequisite. Section 371 conspiracies are punishable by imprisonment for not more than five years. Elsewhere, conspirators often face more severe penalties. These differences aside, federal conspiracy statutes share much common ground because Congress decided they should. As the Court observed in Salinas , "When Congress uses well-settled terminology of criminal law, its words are presumed to have their ordinary meaning and definition. [When] [t]he relevant statutory phrase is 'to conspire,' [w]e presume Congress intended to use the term in its conventional sense, and certain well-established principles follow." These principles include the fact that regardless of its statutory setting, every conspiracy has at least two elements: (1) an agreement (2) between two or more persons. Members of the conspiracy are also liable for the foreseeable crimes of their fellows committed in furtherance of the common plot. Moreover, statements by one conspirator are admissible evidence against all. Conspiracies are considered continuing offenses for purposes of the statute of limitations and venue. They are also considered separate offenses for purposes of sentencing and of challenges under the Constitution's ex post facto and double jeopardy clauses. This is a brief discussion of the common features of federal conspiracy law that evolved over the years, with passing references to some of the distinctive features of some of the statutory provisions. Although it is not without common law antecedents, federal conspiracy law is largely of Congress's making. It is what Congress provided, and what the courts understood Congress intended. This is not to say that conspiracy was unknown in pre-colonial and colonial England, but simply that it was a faint shadow of the crime we now know. Then, it was essentially a narrow form of malicious prosecution, subject to both a civil remedy and prosecution. In the late 18 th and early 19 th centuries, state courts and legislatures recognized a rapidly expanding accumulation of narrowly described wrongs as "conspiracy." The patchwork reached a point where one commentator explained that there were "few things left so doubtful in the criminal law, as the point at which a combination of several persons in a common object becomes illegal." Congress enacted few conspiracy statutes prior to the Civil War. It did pass a provision in 1790 that outlawed confining the master of a ship or endeavoring revolt on board. This, Justice Story, sitting as a circuit judge, interpreted to include any conspiracy to confine the prerogatives of the master of ship to navigate, maintain, or police his ship. The same year, 1825, Congress outlawed conspiracies to engage in maritime insurance fraud. Otherwise, there were no federal conspiracy statutes until well after the mid-century mark. During the War Between the States, however, Congress enacted four sweeping conspiracy provisions, creating federal crimes that have come down to us with little substantive change. The first, perhaps thought more pressing at the beginning of the war, was a seditious conspiracy statute. Shortly thereafter, Congress outlawed conspiracies to defraud the United States through the submission of false claim, and followed that four years later with prohibitions on conspiracies to violate federal law or to defraud the United States. Subsequent conspiracy statutes, though perhaps no less significant, were more topically focused. The Reconstruction civil rights conspiracy provisions, the Sherman Act anti-trust provisions, and the drug and racketeering statutes may be the best known of these. All of them begin the same way—with an agreement by two or more persons. There are no one-man conspiracies. At common law where husband and wife were considered one, this meant that the two could not be guilty of conspiracy without the participation of some third person. This is no longer the case. In like manner at common law, corporations could not be charged with a crime. This too is no longer the case. A corporation is criminally liable for the crimes, including conspiracy, committed at least in part for its benefit, by its officers, employees and agents. Moreover, a corporation may be criminally liable for intra-corporate conspiracies, as long as at least two of its officers, employees, or agents are parties to the plot. Notwithstanding the two-party requirement, no co-conspirator need have been tried or even identified, as long as the government produces evidence from which the conspiracy might be inferred. Even the acquittal of a co-conspirator is no defense, although no conviction is possible if all but one alleged conspirator are acquitted. Moreover, a person may conspire for the commission of a crime by a third person though he himself is legally incapable of committing the underlying offense. On the other hand, two people may not always be enough. The so-called Wharton's Rule placed a limitation on conspiracy prosecutions when the number of conspirators equaled the number of individuals necessary for the commission of the underlying offense. Under federal law, the rule "stands as an exception to the general principle that a conspiracy and the substantive offense that is its immediate end do not merge upon proof of the latter." And under federal law, the rule reaches no further than to the types of offenses that gave birth to its recognition—dueling, adultery, bigamy, and incest. It is not enough, however, to show that the defendant agreed only with an undercover officer to commit the underlying offense, for there is no agreement on a common purpose in such cases. As has been said, the essence of conspiracy is an agreement, an agreement to commit some act condemned by law either as a separate federal offense or for purposes of the conspiracy statute. The agreement may be evidenced by word or action; that is, the government may prove the existence of the agreement either by direct evidence or by circumstantial evidence from which the agreement may be inferred. "Relevant circumstantial evidence [may] include[]: the joint appearance of defendants at transactions and negotiations in furtherance of the conspiracy; the relationship among codefendants; mutual representation of defendants to third parties; and other evidence suggesting unity of purpose or common design and understanding among conspirators to accomplish the objects of the conspiracy." The lower federal appellate courts have acknowledged that evidence of a mere buyer-seller relationship is insufficient to support a drug trafficking conspiracy charge. Some do so under the rationale that there is no singularity of purpose, no necessary agreement, in such cases: "the buyer's purpose is to buy; the seller's purpose is to sell." Others do so to avoid sweeping mere one-time customers into a large-scale trafficking operation. Still others do so lest traffickers and their addicted customers face the same severe penalties. All agree, however, that purchasers may be liable as conspirators when they are part of a larger scheme. Again, in most cases the essence of conspiracy is agreement. "Nevertheless, mere association, standing alone, is inadequate; an individual does not become a member of a conspiracy merely associating with conspirators known to be involved in crime." The task of sifting agreement from mere association becomes more difficult and more important with the suggestion of overlapping conspiracies. Criminal enterprises may involve one or many conspiracies. Some time ago, the Supreme Court noted that "[t]hieves who dispose of their loot to a single receiver—a single 'fence'—do not by that fact alone become confederates: They may, but it takes more than knowledge that he is a 'fence' to make them such." Whether it is a fence, or a drug dealer, or a money launderer, when several seemingly independent criminal groups share a common point of contact, the question becomes whether they present one overarching conspiracy or several separate conspiracies with a coincidental overlap. In the analogy suggested by the Court, spokes with a common hub need an encompassing rim to function as a wheel. When several criminal enterprises overlap, they are one overarching conspiracy or several overlapping conspiracies depending upon whether they share a single unifying purpose and understanding—one common agreement.