Currently there is no legal definition of targeted killings in either international or domestic law. ‘Targeted killing’ is rather a descriptive notion frequently used by international actors in order to refer to a specific action undertaken in respect to certain individuals.
Various scholars propose different definitions. Machon, for example, refers to ‘targeted killing’ as an “intentional slaying of a specific individual or group of individuals undertaken with explicit governmental approval,” whereas Solis suggests that for there to be a targeted killing (i) there must be an armed conflict, either international or non-international in character; (ii) the victim must be specifically targeted; (iii) he must be beyond a reasonable possibility of arrest; (iv) the killing must be authorized by senior military commanders or the head of government; (v) and the target must be either a combatant or someone directly participating in the hostilities. But whereas some scholars seek to use a human rights-based definition,  others propose those which do not entail the applicability of international humanitarian law. 
However, such definitions are incorrect for several reasons. First of all, the definition of a ‘targeted killing’ has to be broad enough as to cover a wide range of practices and flexible enough as to encompass situations within and outside the scope of an armed conflict, thus, being subject to the application of both international human rights law and international humanitarian law, as opposed to the definition provided by some scholars and even states themselves. Secondly, one should bear in mind that defining an act as an instance of ‘targeted killing’ should not automatically render the illegality of such an act at stake. Moreover, the definition also has to cover situations where such an act is carried out by other subjects of international law, rather than only by states.
Therefore, maintaining an element-based approach and synthesizing common characteristics of multiple definitions, it is more advisable to use the one employed by Alston and Melzer, which refers to targeted killings as a use of lethal force by a subject of international law (encompassing non-state actors) that is directed against an individually selected person who is not in custody and that is intentional (rather than negligent or reckless), premeditated (rather than merely voluntary), and deliberate (meaning that ‘the death of the targeted person [is] the actual aim of the operation, as opposed to deprivations of life which, although intentional and premeditated, remain the incidental result of an operation pursuing other aims).
Moreover, being a descriptive notion, ‘targeted killing’ does not entail legal rights and obligations per se. The legality of the actions referred to as an instance of ‘targeted killing’ is rather dependent on the compliance of such an action with the norms of international law applicable to it. As it has been argued before, given its descriptive nature and a broad scope of practices referred to as ‘targeted killings’, the notion is subject to the application of a complex legal framework. The legality of every instance of ‘targeted killing’ is to be assessed on a case-by-case basis, and whether or not a specific targeted killing is legal depends on the context in which it is conducted: in an armed conflict, outside an armed conflict, or in relation to the use of force.
In cases of an armed conflict specific norms of international human rights law seize to apply and are substituted by those of international humanitarian law as lex specialis. Whether a particular targeted killing is legal under international humanitarian law, whose applicability is triggered by the existence of an armed conflict, is determined by several criteria. Firstly, a killing is lawful only when the target is a combatant or a civilian directly participating in hostilities. Additionally, the killing must constitute a military necessity, the use of lethal force must be proportionate to the direct military advantage anticipated, whereas everything feasible must be done to minimize collateral damage and harm to the civilian population. Moreover, these standards apply regardless of the character of an armed conflict.
Whether a particular targeted killing is legal under international human rights law depends on the compliance of the use of lethal force with the requirements of necessity and proportionality. The requirement of necessity implies that there are no other means rather than the use of lethal force in order to prevent the threat to life, whereas proportionality implies that a killing is only legal to protect life. Thus, proportionality limits the permissible level of force based on the threat posed by the suspect to others, whilst necessity imposes an obligation to minimize the level of force used, regardless of the amount that would be proportionate through, for example, the use of warnings, restraint, and capture. 
Therefore, the use of lethal force under international human rights law is legal if it is strictly and directly necessary to save life and, thus, ‘targeted killing’ as a deliberate, intentional and premeditated deprivation of life is illegal under international human rights law with killing itself being a sole objective of an operation, unless, such an operation is intended to save the lives of others.
It is customary international law that States have the right to use force to maintain domestic order.  However, a targeted killing conducted in the territory of another state might violate the principle of sovereignty and amount to aggression, which is prohibited by Article 2(4) of the UN Charter. Nonetheless, there are certain exceptions justifying such an extra-territorial use of force. An extra-territorial targeted killing does not violate the principle of sovereignty either if the state whose sovereignty is at stake consents,  or if the targeting state has a right to use force in self-defense in response to an ‘armed attack’ as provided for by Article 51 of the UN Charter.  The second exception entails the following prerequisites: the second state is either responsible for an armed attack against the first state or the second state is unwilling or unable to stop armed attacks against the first state emanating from its territory. 
Moreover, the use of lethal force in self-defense in response to an ‘armed attack’ must also comply with the principles of necessity and proportionality. Proportionality requires that a state acting defensively employ no more force than reasonably required to overcome the threat. In the context of cross-border operations, this limitation means that the scale and nature of the force employed cannot exceed that which is necessary. 
Article 38(1)(b) of the Statute of the International Court of Justice accepts ‘international custom’ as a source of law,  but only where this custom signifies a general practice which constitutes opinio juris, i.e is accepted as law.  As it has been argued above, ‘targeted killings’ do not have a legal definition and, thus, are not treated as such by states. Given the absence of such a treatment, we can reasonably argue that there is no opinio juris present and that there is, thus, no rule of customary international law emerging. Moreover, the legal framework governing targeted killings contains jus cogens, i.e. peremptory norms from which no derogation is possible and which can be modified only by subsequent norms of general international law of the same character.  What is most important in this context is that only several subjects of international law cannot create jus cogens and thereafter impose their interpretation on the majority of States. 
Therefore, both insignificant contemporary state practice and the absence of opinio juris do not attest the emergence of a rule of customary international law nor do they provide any justification for such actions, which are rather governed by the legal framework described above.
Written by Jan Guardian
 Philip Alston, The CIA and Targeted Killings Beyond Borders , 2 Harv. Nat’l Sec. J. 283 (2011)(hereinafter Alston, Targeted Killings), p. 295.
 Matthew J. Machon, Targeted Killing as an Element of U.S. Foreign Policy in the War on Terror. Fort Leavenworth, KS: School of Advanced Military Studies, 2006, p. 20.
 Gary D. Solis. The Law Of Armed Conflict: International Humanitarian Law in War. Cambridge: Cambridge University Press, 2010 (hereinafter Solis, Law of Armed Conflict), pp. 542-43.
 L. Gross. Moral Dilemmas Of Modern War: Torture, Assassination, and Blackmail in an Age of Asymmetric Conflict. Cambridge: CambridgeUniversity Press, 2010, p. 106.
 Michael 5 Solis, Law of Armed Conflict, supra note 3.
 See e.g., Public Committee against Torture in Israel v. Government of Israel, HCJ 769/02. IsrSC 57(6) (2006), p. 285.
 Alston, Targeted Killings, supra note 1, pp. 297-298.
 U.N. Human Rights Council, Report of the Special Rapporteur on Extrajudicial, Summary or Arbitrary Executions, Addendum: Study on Targeted Killings, U.N. Doc. A/HRC/14/24/Add.6, May 28, 2010 (prepared by Philip Alston), paras. 1, 10; Nils Melzer. Targeted Killing in International Law. Oxford: OxfordUniversity Press, 2008, pp. 3-4 [online][accessed 26 February 2013].
 Alston, Targeted Killings, supra note 1, p. 300.
 Alston, Targeted Killings, supra note 1, p. 301.
 See e.g., Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1996 J.C.J 226, July 8, 1996 [online][accessed 26 February 2013].
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 See e.g., Basic Principles on the Use of Force and Firearms by Law Enforcement Officials, adopted by the Eighth U.N. Congress on Prevention of Crime and Treatment of Offenders, Havana., Cuba, Aug. 27-Sept. 7, 1990.
 U.N. Human Rights Council, Report of the Special Rapporteur on Extrajudicial, Summary, or Arbitrary Executions (prepared by Philip Alston), U.N. Doc. A/61/311, September 5, 2006, pp. 42-44.
 Malcolm Shaw, International Law, 6th ed. Cambridge: Cambridge University Press, 2008, p. 1126.
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 See, e.g., Ian Brownlie, International Law and the Activities of Armed Bands, 7 Int’l & Comp. L. Q. 712, (1958) hereinafter Brownlie, Armed Bands), p. 732.
 UN Charter, supra note 19, art. 51.
 Advisory Opinion Concerning Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, International Court of Justice (ICJ), 9 July 2004, ICJ Rep 136, para 139 [online][accessed 26 February 2013].
 Nils Melzer, Targeted Killing supra note 8, p. 288.
 Armed Activities on the Territory of the Congo (Dem. Rep. Congo v. Uganda), December 19, 2005 , J.C.J. 116, para. 147 [online][accessed 26 February 2013].
 United Nations, Statute of the International Court of Justice, 18 April 1946, art. 38(1)(b) [online][accessed 26 February 2013].
 North Sea Continental Shelf Case (Federal Republic of Germany v. Denmark; Federal Republic of Germany v. Netherlands), International Court of Justice (ICJ), 20 February 1969, ICJ Reports 1969, para. 77 [online][accessed 26 February 2013].
 Alston, Targeted Killings, supra note 1, p. 295.
 M. Cherif Bassiouni, International Crimes: ‘Jus Cogens’ and ‘Obligatio Erga Omnes’. In: Law and Contemporary Problems. Vol. 59, No. 4, p. 68.
 C. Tomuschat, Obligations Arising for States Without or Against their Will, 241 Recueil des Cours (1993), p. 307.