国际法与国内法的关系

国际法与国内法可能产生冲突，当两者冲突时的处理方法就体现了国际法与国内法的关系。一元论和二元论是处理国际法与国内法的关系的两种理论。大部分国家采用了部分一元部分二元的法律体系。
国际法大于国内法.
一元论认为国际法与国内法融为一体，共同用来确定一项行为是否合法。即使有些国家宣称实施一元论，但仍区别对待国际公约和其他国际法（如国际习惯法和）；这说明该国实际采用的是部分一元部分二元的法律体系。
在德国等国家，国际法与国内法具有相同的效力，但德国采用，如果签署国际法后新通过的法律与国际法相抵，则新的国内法有效，国际法无效。
如果采用纯一元体系，国际法一旦被签署就立即在签署国生效，习惯法也在该国有效。国内法院和民众都可以直接援引国际法，如同援引国内法一样。如果国内法与国际法相抵触，国内法就会失效，不论是普通法还是宪法，不论是新法或旧法。
从人权角度看，一元体系有其优点。假设一个国家签署了《公民权利和政治权利国际公约》，但国内法限制了言论自由，民众因言获罪时便可援引国际公约，要求法官根据国际公约宣判其无罪，并宣布限制言论自由的国内法无效。毕竟，热衷言论管制的国家不太愿意按照签署的人权公约制定相应的国内法。签署人权公约可能只是迫于政治压力或讨好国际社会。
国内法大于国际法.
二元体系强调国际法和国内法的区别，国际法需要借以国内法来实施。虽然签署国际公约但不实施该公约是有违国际法的，但该公约还是不属于该国的法律，不可用于判案。与国际法抵触的国内法仍旧有效。根据二元论，法官从不援引国际法，但国内法可以按照国际法修改或指定。
"International law as such can confer no rights cognisable in the municipal courts. It is only insofar as the rules of international law are recognized as included in the rules of municipal law that they are allowed in municipal courts to give rise to rights and obligations".
The supremacy of international law is a rule in dualist systems as it is in monist systems. Sir Hersch Lauterpacht pointed out the Court's determination to discourage the evasion of international obligations, and its repeated affirmation of:
If international law is not directly applicable, as is the case in monist systems, then it must be translated into national law, and existing national law that contradicts international law must be "translated away". It must be modified or eliminated in order to conform to international law.
Again, from a human rights point of view, if a human rights treaty is accepted for purely political reasons, and states do not intend to fully translate it into national law or to take a monist view on international law, then the implementation of the treaty is very uncertain.
Examples.
In some countries, such as the UK for instance, the dualist view is predominant. International law is only part of British national law once it is accepted in national law. A treaty 
The United States of America has a "mixed" monist-dualist system; international law applies directly in US courts in some instances but not others. US Constitution, art. VI, does indeed say that treaties are part of the Supreme Law of the Land, as suggested by the quote above; however, its Supreme Court, as late as the recent case of Medellín v. Texas, has restated that some treaties are not "self-executing." Such treaties must be implemented by statute before their provisions may be given effect by national and sub-national courts. Similarly with regard to customary international law, its Supreme Court stated, in the case of the Pacquete Habana (1900), that "international law is part of our law." However, it also said that international law would not be applied if there is a controlling legislative, executive, or judicial act to the contrary...
A matter of national legal tradition.
International law does not determine which point of view is to be preferred, monism or dualism. Every state decides for itself, according to its legal traditions. International law only requires that its rules are respected, and states are free to decide on the manner in which they want to respect these rules and make them binding on its citizens and agencies. 
“he transformation of international norms into domestic law is not necessary from the point of view of international law…the necessity of transformation is a question of national, not of international law”.
Both a monist state and a dualist state can comply with international law. All one can say is that a monist state is less at risk of violating international rules, because its judges can apply international law directly. Negligence or unwillingness to implement international law in national law can only pose a problem in dualist states. 
States are free to choose the way in which they want to respect international law, but they are always accountable if they fail to adapt their national legal system in a way that they can respect international law. Either they adopt a constitution that implements a monist system so that international law can be applied directly and without transformation, or they do not. But then they have to translate all international law in national law. 
In a monist state we rely only on the judges and not on the legislators, but judges can also make mistakes. If a judge in a monist states makes mistakes when applying international law, then the country violates international law just as much as a dualist country that, for one reason or another, does not allow its judges to apply international law directly and fails to translate or fails to translate correctly and effectively. One reason for preferring dualism is precisely the fear that national judges are not familiar with international law - a highly complex field of law - and hence are liable to make mistakes.
The problem of “lex posterior”.
In dualist systems, international law must be translated into national law, and existing national law that contradicts international law must be "translated away". It must be modified or eliminated in order to conform to international law. However, the need for translation in dualist system causes a problem with regard to national laws voted after the act of translation. 
In a monist system, a national law that is voted after an international law has been accepted and that contradicts the international law, becomes automatically null and void at the moment it is voted. The international rule continues to prevail. In a dualist system, however, the original international law has been translated into national law - if all went well - but this national law can then be overridden by another national law on the principle of "lex posterior derogat legi priori", the later law replaces the earlier one. This means that the country - willingly or unwillingly - violates international law. A dualist system requires continuous screening of all subsequent national law for possible incompatibility with earlier international law.