“Federal criminal law,” one of my law partners muttered in the mid-1980s, “is becoming silly putty.” A quarter century later, her passing complaint looks more like prophecy. At the start of the 21st century, we are in an age when honest citizens are at risk of federal indictment for professional conduct widely considered proper.

This development has dangerously eluded legislators, judges, lawyers, and professionals in industry, commerce, the arts, and the press. So the debates over the guilt or innocence of high-and-low profile professionals indicted by the Department of Justice, from corporate finance innovator Michael Milken to the now-defunct Arthur Andersen accounting firm to, most recently, newspaper magnate Conrad Black, have proceeded on a variety of false premises.

What these defendants actually did is fairly well-known. What should be debated is why, under existing statutes, their actions were deemed criminal. The common thread is the ever-increasing body of vague federal criminal statutes and turgid regulations. These laws, which the Department of Justice seems content to keep in their most malleable form, explain why so many targets of federal prosecutors in recent decades have seemed perplexed and outraged over prosecutions, convictions, or coerced guilty pleas, and long prison sentences growing out of conduct they deemed lawful.

Milken was coerced into pleading guilty to a half-dozen counts of “securities fraud” when prosecutors agreed, in exchange, to drop charges against his equally innocent younger brother. A Milken associate later went to trial in one of the cases in which Milken had already pleaded guilty, and the trial judge acquitted the associate on the ground that the transaction simply was not a crime.

Likewise, federal prosecutors indicted Arthur Andersen & Company for “obstruction of justice” growing out if its treatment of audit work papers from its Enron engagement. The firm was following its own wholly appropriate document retention-and-destruction policy when it shredded boxes of material from the audit — and ceased doing so the moment a subpoena arrived.

The indictment nearly put the firm out of business, and the conviction finished the job. Andersen had only a few hundred employees left — their task was the orderly wrap-up of the defunct firm — when the Supreme Court in 2005 unanimously reversed the conviction. In one of legal history’s Pyrrhic victories, the high court concluded that the trial judge allowed the jury to convict even if the firm had committed no crime.

Philip Russell, a lawyer in Greenwich, Conn., was pressured into pleading guilty to misprision of a felony — knowing someone committed a felony but failing to report it — in 2007. The respected local attorney destroyed the hard drive of a computer owned by his client, a local church, after he was informed that a church employee had put child pornography on the machine. The mere possession of such images is a serious federal crime, and Russell did what many experienced lawyers would have done — destroyed what the law classified as prohibited contraband. For that, he was pressured into a plea bargain that kept him out of prison but cost him his license.

Steven Kurtz, artist and faculty member at the State University of New York at Buffalo, was indicted in 2003 for “mail fraud” for engaging in a transaction that academics around the country engage in routinely. The fraud charge was simply a way for the government to justify the countless man-hours the FBI poured into the investigation after bizarrely concluding that Kurtz’s cutting edge artwork, using harmless bacteria mixed in with his paint medium, amounted to bioterrorism. Kurtz, one of the lucky ones, refused to knuckle under and was recently exonerated.

The list goes on. Virtually any professional, engaging in seemingly legitimate practices, can find himself in a Kafkaesque nightmare where effective defense is nearly impossible, as the various federal anti-fraud and obstruction-of-justice statutes have neither clear definition nor logical bounds. It is no surprise that Conrad Black’s recent conviction provoked controversy. Black was indicted in 2005 after selling Hollinger International, the newspaper empire he headed, and allegedly defrauding the shareholders of a subsidiary.

Although acquitted of a majority of the charges, Black was convicted of mail and wire fraud — the use of these means of interstate commerce for the purpose of effectuating a fraud. The federal court of appeals in Chicago, in a breezy and often sarcastic opinion, affirmed his conviction and 78-month prison sentence this past June.

A mere 20 days after oral argument concerning the complex transaction, the court blithely concluded that Black’s conduct could be swept within the fraud statute’s notion of “a scheme or artifice to defraud” because it involved depriving Hollinger’s shareholders “of their intangible right to the honest services of the corporate officers.” There is no accepted definition, even among courts, as to what this means. Yet a transaction that many in good faith defend was deemed a felony.

Before prosecutors and courts convict and imprison professionals who go about their daily business, the law should be made sufficiently clear so that the well-meaning citizen can conform his conduct to legal requirements. Without such lucidity, the law becomes a trap for the unwary, and the average law-abiding citizen will go about his business, unaware that between morning and night, he may well have committed multiple arguable felonies.

On most days, and for most people, these will go unnoticed. But within the crosshairs of ambitious prosecutors, subject to inadequate controls by an often feckless judiciary that pretends to understand these statutes (much as the deluded emperor imagined himself wearing magnificent garb), one will find firsthand that “liberty and justice” have become “silly-putty.”

Mr. Silverglate, a Cambridge, Mass., criminal defense and civil liberties trial lawyer, is the author of the forthcoming “Three Felonies a Day: How the Feds Target the Innocent,” to be published next year by Encounter Books. The author was one of Michael Milken’s lawyers.