Obstruction of justice, in a common law state, refers to the crime of offering interference of any sort to the work of police, investigators, regulatory agencies, prosecutors, or other (usually government) officials. Often, no actual investigation or substantiated suspicion of a specific incident need exist to support a charge of obstruction of justice.

Generally, obstruction charges are laid when it is discovered that a person questioned in an investigation, who is not a suspect, has lied to the investigating officers. However, in most common law jurisdictions, the right to remain silent allows any person who is questioned by police merely to refuse to answer questions posed by an investigator without giving any reason for doing so. (In such a case, the investigators may subpoena the witness to give testimony under oath in court) It is not relevant if the person lied to protect a suspect (such as setting up a false alibi, even if the suspect is in fact innocent) or to hide from an investigation of their own activities (such as to hide their involvement in another crime). Obstruction charges can also be laid if a person alters or destroys physical evidence, even if they were under no compulsion at any time to produce such evidence.

Some famous examples:

• Martha Stewart was convicted of obstruction of justice, as well as other charges, for falsifing her trading records after learning her friend Sam Waksal , the CEO of Imclone , the company in whose stock she sold, was being investigated for insider trading. It appears that Ms. Stewart's own trading activities did not meet the strict definition of insider trading, and the falsification of documents was intended merely to create an explanation for what was a suspicious trade. However, her actions made it more difficult to prove that Ebbers had also sold his stock in anticipation of negative news of the lack of FDA approval for Imclone's product.
• President Richard Nixon was being investigated for obstruction of justice for his alleged role in the cover-up of the break-in at the Watergate hotel during his 1972 re-election campaign. Although it is widely believed that Nixon had no foreknowledge of his re-election committee's "dirty tricks" campaign against Democrat presidential candidates that led to the break-in, it appears he was aware of it after the fact and planned to pay money to keep the participants quiet.
• An insider trader instructed his banker in the Cayman Islands to shred records of his suspicious trades when he learned he was under investigation by the U.S. Securities and Exchange Commission. Unfortunately, this set up the banker for a charge of obstruction of justice, and the U.S. Department of Justice traded immunity for the obstruction charge in exchange for the banker's testimony about the trades he executed on behalf of the trader. Ironically, the banker could merely have refused to produce the trading records when asked, hiding behind the Cayman Islands' bank secrecy laws without being guilty of obstruction.