This thesis evaluates the justifiability of preparatory offences: offences that target acts of preparation carried out with the intent to commit some further crime. The preventive turn in the criminal law has seen, amongst other changes, a broadening in the scope of preparatory offences. New offences reach far back into the early stages of preparation, targeting conduct that, but for the criminal intent, would often be entirely innocuous. Notable examples arise in the terrorist and child-sex offending contexts. Section 5 of the Terrorism Act 2006 criminalises any conduct in preparation for giving effect to an intention to commit acts of terrorism. Section 15 of the Sexual Offences Act 2003 criminalises communicating with a person under 16 and arranging to meet them, with the intention of committing a sexual offence against them in the future. The breadth of these offenses, and their focus on criminal intent, has led to concerns that they are akin to 'thought crimes' and thus represent an oppressive overreach of state power. This thesis begins by reviewing current theoretical thinking on the limits of the criminal law. It contributes to existing liberal accounts of the limits of the criminal law by exploring in more detail the specific issues raised by preparatory offences, structured as they are around an ulterior intent. The methodology will involve considering the familiar inchoate offences of attempt, conspiracy and possession with intent and the debates pertaining to their justifiability and scope. By engaging with the literature on these offences, this thesis moves the debate on the liberal limits of the criminal law forward by refining the constraints on criminalisation already identified by other theorists. More importantly, this thesis begins the difficult work of demonstrating how those constraints should be applied to specific offences: attempts, conspiracy and compound possession offences.