What does Article 7 cover?
Non-retroactivity, the principle of legality or, if you are into Latin, nullum crimen nulla poena sine lege (no crime, no punishment without law).
And precisely what does Article 7(1) say exactly?
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 that it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the criminal offence was committed.
How might an issue under this Article arise?
Two cases reached the European Court of Human Rights in relation to the imposition of compensation orders on those convicted of certain criminal offences, notably drug trafficking. Mr Welch was convicted of drug offences in 1986. Compensation orders were only introduced in January 1987 when the Drug Trafficking Offences Act 1986 came into force. He successfully argued that the order was a retrospective penalty. ((Welch v UK (1995) 20 EHRR 247 )) Mr Taylor was not so lucky. He was convicted of offences in the early 1990s but the confiscation order took into account offences back to the 1970s. He was deemed to have been able to know the potential consequences of his actions. ((Taylor v UK  EHRLR 90)) Similarly unsuccessful was an appellant who argued that the article protected him from changes of law made after offences were committed that meant he would have to serve a longer minimum prison term before early release for good behaviour because convicted only some time after the offence. His maximum term was actually a life sentence and he got 12 years. The House of Lords accepted that the article would only have been breached if he was asked to serve a longer sentence than it was possible to receive at the time of the offence. ((R (Uttey) v Home Secretary  1 WLR 2278
But how does the article apply if judges devise a new common law offence? Has this always existed or is it newly created? The argument is stymied a bit by the House of Lords finally determining that this previously asserted power (as in the creation of the offence of conspiracy to corrupt public morals, ((Shaw v DPP  AC 220))) no longer exists. ((Knuller (Publishing, Printing and Promotions) Ltd v DPP  AC435)) A related point arose, however, in relation on the determination of the courts that rape could exist within a marriage whereas previously it had been the common law that consent was implied. The courts dealt with this by distinguishing the point: ((R v R  1 AC 599, SW v UK, C v UK (1995) EHRR 404))
This is not the creation of a new offence, it is the removal of a common law fiction which has become anachronistic and offensive.
Governments cannot derogate from this provision. An exception exists in Article 7(2):
This article shall not prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according to the general principles of law recognised by civilised nations.
This was introduced specifically to cover measures such as those introduced after the Second World War to punish war crimes, treason or collaboration. The exception was used to uphold legislation in Belgium and Denmark under which collaborators were retrospectively criminalised. ((Eg App 214/56 De Becker v Belgium, 9 June 1958)) The phrase ‘general principles of law recognised by civilised nations’ is a reference to Article 38 of the Statute of the International Court of Justice.