Illegal or prohibited acts are categorized either according to the body of humanitarian law they violate or consequences for the perpetrator. Some acts involve prohibited means or methods of warfare (“Hague law,” that is, the law arising from the Hague Conventions of 1899 and 1907). Other acts harm protected persons—sick and wounded, shipwrecked, or civilians ( "Geneva law," that is, the law arising from the Geneva Conventions).

The most serious of illegal acts are grave breaches of the Geneva Conventions of 1949. Illegal acts, other than grave breaches, are known as serious violations and, unlike grave breaches, are not subject to universal jurisdiction, although they may and often will be subject to prosecution in different courts, including international tribunals.

Illegal acts include: use of prohibited means and methods of warfare, including poison or other weapons calculated to cause unnecessary suffering; perfidious attacks not involving the abuse of protective emblems or emblems or uniforms of neutral countries; failure to wear a uniform to identify oneself as a legal combatant; pillage; terrorism; interference with humanitarian aid shipments; nonextensive, unjustified destruction of property; attack or bombardment of undefended towns, dwellings, or buildings; seizure of or willful damage done to certain cultural institutions, e.g., those dedicated to religion, education, charity, arts, sciences, or historic monuments and works of art; reprisals against protected persons or objects; and any breach of an armistice.

For its own purposes, the International Criminal Tribunal for the Former Yugoslavia defined a serious violation as one that has grave consequences for its victim and which breaks a rule protecting important values. Unnecessarily burning a village’s crop is serious; stealing a loaf of bread is not.

Regardless of whether an individual must be punished, parties to a conflict are responsible for all violations of international humanitarian law.

(See war crimes.)