The “principle of legality” (Nullum crimen sine lege no crime without law) requires that prosecution and punishment be based upon clear provisions of international law, at the time the crime was committed. This principle has been considered as a customary international humanitarian law by the ICRC. Now, the strict principle of legality contains four sub-rules: specificity, non-retroactivity, the ban on analogy, and favouring the accused. As for specificity, it requires the definition of crimes to be sufficiently clear and precise. Non-retroactivity refers that an individual should not be prosecuted for his/her behaviour not specifically criminalised by law when it was committed. The rule of non-retroactivity was also embedded in many human rights treaties as non-derogable, including the ICCPR and article 7 of the European Convention on Human Rights. The rules of banning on analogy and favouring the accused are also limitations of judicial interpretations. A question arises whether the definition of the crimes against humanity under article 7 of the Rome Statute with open-ended crimes, such as “other inhumane acts”, is a violation of the principle of legality.
As Cassese observed, international criminal law is changing, and this legal change is accompanied by a gradual shift from the doctrine of substantive justice to that of strict legality. In the Nuremberg IMT, judges found that this principle is not absolute and should give way to the principle of justice. They stated that
“the maxim nullum crimen sine lege is not a limitation of sovereignty, but is in general a principle of justice. To assert that it is unjust to punish those who in defiance of treaties and assurances have attacked neighbouring states without warning is obviously untrue, for in such circumstances the attacker must know that he is doing wrong, and so far from it being unjust to punish him, it would be unjust if his wrong were allowed to go unpunished.”
This argument subsequently has been employed by the ICTY to justify their judicial development of war crimes in internal armed conflicts, as it warranted the “substantive justice and equity”. It shows a flexible application of this principle (as we understand now) and the principle of justice is more important than the principle of legality.
Another viewpoint of the principle of legality stressed the prohibition of retroactivity. Justice Robertson expressed that
“[t]he principle of legality, sometimes expressed as the rule against retroactivity, requires that the defendant must at the time of committing the acts alleged to amount to a crime have been in a position to know, or at least readily to establish, that those acts may entail penal consequences. Ignorance of the law is no defence, so long as that law is capable of reasonable ascertainment. The fact that his conduct would shock or even appal decent people is not enough to make it unlawful in the absence of a prohibition. The requisite clarity will not necessarily be found in there having been previous successful prosecutions in respect of similar conduct, since there has to be a first prosecution for every crime and we are in the early stages of international criminal law enforcement. Nor is it necessary, at the time of commission, for there to be in existence an international court with the power to punish it, or any foresight that such a court will necessarily be established. In every case, the question is whether the defendant, at the time of conduct which was not clearly outlawed by national law in the place of its commission, could have ascertained through competent legal advice that it was contrary to international criminal law. ”
Similarly, the European Court of Human Rights (ECtHR) construed that this principle allows courts to clarify rules through judicial interpretation case-by-case. The ECtHR formulated two tests, i.e., foreseeability and accessibility, to assess whether there is a violation of this principle.  It does not matter whether the law is written or unwritten. This principle is satisfied “where the individual can know from the wording of the relevant provision and, if need be, with the assistance of the courts’ interpretation of it, what acts and omissions will make him criminally liable.”
The principle of strict legality is also outlined in articles 22, 24(1) and 24(2) of the Rome Statute. The reference to “a crime within the jurisdiction of the Court” in article 22(1) aimed to exclude any possibility of prosecution for offences merely based on customary law by the ICC. In addition, Article 24 in the title of “non-retroactivity ratione personae” further limited the temporary jurisdiction of the ICC. It means that although the Rome Statute was adopted in 1998, the ICC has no temporary jurisdiction over crimes committed from 1998 to 2002 until the entry into force of the Rome Statute. Hence, the Rome Statute provides an “unprecedented” emphasis on the principle of legality.
Despite the unprecedented emphasis, the strict legality principle does not exclude the possibility that the definitions of crimes in the Rome Statute are never ambiguous. Commentators and case law have shown that the principle of legality does not bar the progressive application and formulation of customary international law by international tribunals. This principle is only a limitation for the interpretation of customary international law. It is also true for treaty law. Pursuant to article 7 of the Rome Statute, the scope of “other inhumane acts” is limited to be “of a similar character intentionally causing great suffering, or serious injury to body or to mental or physical health”. In confirmation the charge of other inhuman acts, Pre-Trial Chamber I referred to article 22 of the Rome Statute, defining the offence as ‘serious violations of international customary law and the basic rights pertaining to human beings, drawn from the norms of international human rights law, which are of a similar nature and gravity to the acts referred to in article 7(1) of the Statute’. Additionally, the Chamber found that “the Rome Statute contains certain limitations, as regards to the action constituting an inhumane act and the consequence required as a result of that action”. 
The argument of Justice Robertson and case law of the ECtHR also guide the interpretation of the principle of legality. The tests of foreseeability and accessibility do not require offence specifically defined. On the one hand, it is difficult for the drafters of the Rome Statute to imagine how cruelty and inhumanity perpetrators would be in the future and to exhaustively list all acts. Only if it is accessible and foreseeable to the perpetrator, the open-ended crime with a residual definition is not a violation of the principle of specificity. On the other hand, new strategies, tools, and weapons are emerging. Only if perpetrators are foreseeable or should have known that the use of them to cause similar or even worse sufferings would be a crime, this open-ended crime of other inhuman acts also is not a violation of non-retroactivity.
In conclusion, the principle of legality is not an absolute principle in international criminal law. Before the Nuremberg IMT, it was not superior to the principle of substantive justice. For the ICTY, it is also not a limitation to the identification and application of customary international law of war crimes in internal armed conflicts. Since negotiators of the Rome Statute might be the main potential defendants before the ICC, it is not difficult to understand why they emphasized the principle of strict legality, in particular, the rule of favouring of the accused. In order to avoid abusing the rule of law (including international law) by governments, judicial interpretations should not go too far. According to this principle, the court is “prevented from creating new law or interpreting existing law beyond the reasonable limits of acceptable clarification.” However, “provided that the resultant development is consistent with the essence of the offence and could reasonably be foreseen”, the judicial interpretation is acceptable and the principle of legality is not violated.
 Jean-Marie Henckaerts and Louise Doswald-beck (eds), Customary International Humanitarian Law, vol 1 Rules, Rule 101 (ICRC and Cambridge University Press 2009). “No one may be accused or convicted of a criminal offence on account of any act or omission which did not constitute a criminal offence under national or international law at the time it was committed….”
 Antonio Cassese [et al], Cassese’s International Criminal Law (3rd ed., Oxford University Press, 2013) 27-36.
 International Covenant on Civil and Political Rights, (1976) 999 UNTS 171, article 15; Convention for the Protection of Human Rights and Fundamental Freedoms (ECHR), (1955) 213 UNTS 221, article 7.
 Article 22(2) of the Rome Statute. The rule of favouring the accused is also related to the evaluation of evidence.
 Antonio Cassese [et al], Cassese’s International Criminal Law, 9.
 Judgment and Sentence of the Nuremberg IMT, p.219.
 Prosecutor v. Tadić, Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction, ICTY-94-1-AR72, 2 October 1995, paras. 135; Prosecutor v. Milan Milutinovic et al., Decision on Dragoljub Ojdanic’s Motion Challenging Jurisdiction-Joint Criminal Enterprise, IT-99-37-AR72, 21 May 2003, para. 37.
 Antonio Cassese [et al.], Cassese’s International Criminal Law,25.
 Prosecutor v. Norman, Decision on Preliminary Motion Based on Lack of Jurisdiction (Child Recruitment) SCSL-2004-14-AR72(E)(31 May 2004), Dissenting Opinion of Justice Robertson, para. 13.(Emphasis added)
 SW v. the UK, ( Judgment), ECtHR Application No. 20166/92) (22 November 1995), para. 36.
 Kokkinakis v. Greece, ECtHR, para. 52; Baranowski v Poland, (Judgment), ECtHR Application No. 28358/95 (First Section) (28 March 2000), para. 55.
 SW v. the UK, ECtHR, para. 35.
 “A person shall not be criminally responsible under this Statute unless the conduct in question constitutes, at the time it takes place, a crime within the jurisdiction of the Court.” William A Schabas, The International Criminal Court: A Commentary on the Rome Statute (Oxford University Press, 2010) 407.
 Claus Kreß, ‘The International Criminal Court as a Turning Point in the History of International Criminal Justice’, Antonio Cassese, ed., The Oxford Companion to International Criminal Justice, (Oxford University Press, 2009), pp. 145-146.
 SW v. the UK, ECtHR, para. 36; Rutaganda Trial Judgement, para 86; Prosecutor v. Milutinović et al., ICTY-99-37-AR72(21 May 2003) paras 37-8; Prosecutor v. Mucić et al., (Judgement) ICTY-96-21-A (20 February 2001) para 173; Nahimana et al. v. The Prosecutor, Judgement, ICTR-99-52-A (28 November 2007), partly dissenting opinion of Judge Shahabuddeen, para 19; Robert Cryer and others, An Introduction to International Criminal Law and Procedure (Cambridge University Press 2014) 17–19; Larrisa J Van Den Herik, The Contribution of the Rwanda Tribunal to the Development of International Law, (Martinus Nijhoff Publishers 2005) 213–14.
 Prosecutor v. Katanga et al., ICC-01/04-01/07-717, Decision on the confirmation of charges, 30 September 2008, para. 448.
 Prosecutor v. Katanga et al., ibid, para. 449.
 Prosecutor v. Hadžihasanović et al. Decision on Interlocutory Appeal Challenging Jurisdiction in Relation to Command Responsibility, IT-01-47-AR72, 16 July 2003, para. 34; Prosecutor v. Milan Milutinovic et al., ICTY-99-37-AR72, 21 May 2003, para 38.
 Prosecutor v. Milan Milutinovic et al., ICTY-99-37-AR72, 21 May 2003, para 38.
 SW v. the UK, ECtHR, para. 35.