This chapter analyses the opposition’s contribution to scrutiny of government bills and policy-making at Westminster. Westminster is classically seen as dominated by an adversarial ‘opposition mode’, giving opposition politicians little policy impact. This chapter argues that there are actually several distinct ‘opposition modes’, which can be more consensual. It summarizes existing literature on opposition, and sets out the basics of how opposition parties are organized in both chambers. The bulk of Westminster legislative amendments are proposed by opposition parliamentarians, and the motivations behind these are explored. Many amendments are actually driven by objectives other than policy change—including seeking government explanations, and embarrassing the government. Hence it is important not to overstate ‘failure’ of such amendments. In addition, many opposition amendments go on to trigger government concessions, particularly via the House of Lords. In various ways, the opposition at Westminster is hence more influential on policy than is often assumed.