The modern definitions of the inchoate offences are well known in the criminal law since the Middle Ages both in European-Continent legal systems and in the English Common Law. They were mostly developed by the case-laws of the Star Chamber Court, which was abolished in 1640. The inchoate offences include three basic offences: Attempt, Conspiracy and Solicitation, but different legal systems defined some other offences as inchoate, such as the Aider and Abettor in Britain after the full validation of the Serious Crimes Act, 2007, c.27, s.44. In this paper it is argued, that there is a very intensive interaction between the fight against terrorism and inchoate offences. The fight against terrorism effects the definition of inchoate offences, and inchoate offences are used as a major instrument of criminal law in the legal fight against terrorism.