Section 51 of the Police Act of 1964 (Great Britain) declares that any person who assaults a constable in the execution of his duty shall be guilty of an offense. However, citizens are generally viewed as having the right to resist an unlawful arrest. Two connected lines of argument support this position. A police officer who acts outside the limits of his authority is not 'in the execution of his duty.' In addition, resistance (using no more force than is reasonable in the circumstances) to unauthorized police activity does not constitute an assault. In practice, the effect of resistance is often dependent upon considerations which are not only unknown to citizens at the time of their actions, but which may also be impossible for them to ascertain. The grounds upon which a purported arrest may be declared unlawful fall roughly into two categories of defect, where the arrest may be substantively or procedurally defective. For the purposes of a lawful arrest, the guilt or innocence of a suspect is, in most cases, immaterial. The officers must have a reasonable suspicion that an individual has committed an arrestable offense, must identify themselves as police officers, and must explain why they are executing the arrest. A number of court cases demonstrate that the enquiries that result from cases of resistance aid citizens very little in determining the circumstances under which a clear decision to resist may be drawn. It is suggested that courts should concentrate upon the whole of the event and attempt to consider the situations of both parties and the reasonableness of their positions in the circumstances known to them. Thus, the basis for responsibility for the offense of resisting could be brought more into line with the bases for criminal responsibility in other offenses. Sample situations are cited; footnotes are included.