This chapter examines how far the police are, and should be, allowed to infringe the freedom of the individual through arrest. It considers the legal rules that the police must follow when deciding to, and during, arrest, as well as their effectiveness in controlling the use of this power. This chapter considers the purpose of arrest and what reasons for arrest are lawful. The use of arrest in the context of suspected terrorism is explored, and ‘citizen arrest’ is also evaluated. Discussion about how the police use their discretion when exercising the power of arrest is situated in our understanding of police ‘working rules’. The chapter shows that arrest is used for many purposes, some more legitimate than others.