provides a fairer vehicle for suspected FTFs to know and respond to allegations against them, compared to the application of administrative or executive measures that can have just as serious rights consequences and punitive effects (see section 3.4). But the fairness, legitimacy and effectiveness of criminal law responses depends on the existence of a rule of law framework that is consistent with fundamental principles of criminal law and international human rights law, which are often in jeopardy in this context. In their counter-terrorism efforts, states have increasingly sought to use criminal law preventively – by criminalizing conduct arising before a terrorist crime is committed (i.e., preparatory acts, and acts deemed to support or contribute to terrorism, such as financing, providing material support or inciting terrorism directly or indirectly). The development of specific legislation on FTFs and prosecutions in practice take this trend a step further. Much legislation now criminalizes travelling or the attempt to travel as preparatory acts, as well as facilitating or supporting the travel of another individual. The result is that a broad array of preparatory acts, as well as forms of facilitation or support for such acts and funding or association with or expression of support for individuals associated with them, are now criminalized in legislation across the OSCE area and beyond. The preventive role of criminal law is not itself new or inherently problematic. It is well established, for example, that inchoate acts such as attempts to commit a crime, direct and public incitement, some preparatory acts, or conspiracy or association with criminal intent, may justify the early intervention of criminal law before any terrorist act has taken place. But there are also limits to the preventive application of criminal law, which are enshrined in basic principles of criminal and human rights law and set out below. The expansion of criminal law – alongside shifts in principles, procedures and penalties – raises questions regarding broader implications for the protection of human rights and its effectiveness in terms of terrorism prevention. The Principle of Legality: Clear and Precise Definitions of Offences The requirement of legality and certainty in criminal law (nullum crimen sine lege or nulla poena sine lege) is enshrined in Article 15 of the International Covenant on Civil and Political Rights (ICCPR) and other human rights instruments.104 Non-retroactivity, certainty, precision and foreseeability are all aspects of the right, and are essential rule of law constraints.105 The principle of lex certa requires that criminal law must be sufficiently clear to allow those within a state’s jurisdiction to understand the law’s limits and modify their behaviour. Human rights treaties explicitly proscribe derogation from the 104  Article 15(1) ICCPR provides: “No one shall be held guilty of any criminal offence on account of any act or omission, which did not constitute a criminal offence, under national or international law, at the time when it was committed. Nor shall a heavier penalty be imposed than one that was applicable at the time when the criminal offense was committed. If, subsequent to the commission of the offence, provision is made by law for the imposition of the lighter penalty, the offender shall benefit thereby.” See also Article 11(2) UDHR; Article 7(1) ECHR; Article 9 of the American Convention on Human Rights (ACHR); see also Articles 22 (Nullum crimen sine lege) and 23 (Nulla poena sine lege) of the Rome Statute of the International Criminal Court (ICC). 105  A. Ashworth and L. Zedner, Preventive Justice, (Oxford: Oxford University Press, 2014), pp. 113-114, (hereafter, A. Ashworth and L. Zedner, Preventive Justice ). Guidelines for Addressing the Threats and Challenges of “Foreign Terrorist Fighters” 35