Article 4 | Anti-slavery
(1) No one shall be held in slavery or servitude.
(2) No one shall be required to perform forced or compulsory labour.
(3) For the purposes of this Article “forced or compulsory labour” shall not include:
(a) any work required to be done in the ordinary course of detention imposed according to the provisions of Art.5 of the Convention or during conditional release from such detention.
(b) any service of a military character, or in the case of conscientious objectors in countries where they are recognised, service exacted instead of compulsory military service.
(c) any service exacted in case of an emergency or calamity threatening the life or well-being of the community.
(d) any work or service which forms part of normal civic obligations.
This Article admits of no derogations or limitations in respect of slavery or servitude. Forced labour may, however, be imposed in circumstances set out in Art.4(3).
The corresponding article 5 of the EU Charter of Fundamental Human Rights provides:
(1) No one shall be held in slavery or servitude
(2) No one shall be required to perform forced or compulsory labour
(3) Trafficking in human beings is prohibited.
This is clearly a necessary update to Article 4 ECHR, since human trafficking is a far more pressing problem in the modern world than slavery (insofar as the two can be distinguished). It covers organised crime including the lucrative trade in illegal immigration as well as networks of sexual exploitation. This is well within the competence of the EU since the implementation of Directive 2011/36 on preventing and combating trafficking in human beings and protecting its victims. On the other hand, Article 51 limits Article 5, like the other rights in the EU Charter, to actions against EU institutions and Member States when they are implementing EU law.
Recently the Strasbourg Court has ruled that trafficking in human beings, although not epxlicitly mentioned in the ECHR, falls within the scope of Article 4: Rantsev v. Cyprus and Russia (2010). In this case, the Court laid down the obligations of governments under Article 4 in the modern climate of trade in humans:
… in addition to criminal law measures to punish traffickers, Article 4 requires member States to put in place adequate measures regulating businesses often used as a cover for human trafficking. Furthermore, a State’s immigration rules must address relevant concerns relating to encouragement, facilitation or tolerance of trafficking.
The Court has stated that it considered it “appropriate” to examine
the extent to which trafficking itself may be considered to run counter to the spirit and purpose of Article 4 of the Convention such as to fall within the scope of the guarantees offered by that Article without the need to access which of the three types of proscribed conduct are engaged by the particular treatment in question. 
Until recently however the most active part of the ECHR provision is Art.4(2). There are two elements in the definition of forced labour derived from International Labour Organisation Convention No. 29 (1930) (incorporated into Convention jurisprudence in X v Federal Republic of Germany (1974) 17 YbK 148):
(1) the work must be performed involuntarily;
(2) the requirement to do the work must be unjust or oppressive or the work itself involve avoidable hardship.
In Silidian v France the European Court of Human Rights upheld an application by a Togolese girl who had been held as an unpaid domestic worker in Paris for four years where she had been deprived of her passport and required to work for fifteen hours a day. Her argument was that France had failed in its positive obligation to have adequate criminal offences in place to prevent such servitude, and the Court agreed that her rights under Article 4(2) had been violated, although they rejected her argument under Article 4(1):
Although the applicant was, in the instant case, clearly deprived of her personal autonomy, the evidence does not suggest that she was held in slavery in the proper sense, in other words that Mr and Mrs B. exercised a genuine right of legal ownership over her, thus reducing her to the status of an “object”. 
(App No. 73316/01An early authority on forced labour was Iverson v Norway (1963) 6 YbK 278. The applicant was convicted after abandoning his post in the public dental service in a northern district which he had been required to take up under Norwegian legislation for a year. The European Commission of Human Rights held that the public dental service ordinance had not imposed forced labour for the purposes of Art.4 since it did not fulfil the conditions set out by the ILO (above). The work was “for a short period, provided favourable remuneration and did not involve any discriminatory, arbitrary or punitive application”.