A well kill is the operation of placing a column of heavy fluid into a well bore in order to prevent the flow of reservoir fluids without the need for pressure control equipment at the surface. It works on the principle that the hydrostatic head of the "kill fluid" or "kill mud" will be enough to suppress the pressure of the formation fluids. Well kills may be planned in the case of advanced interventions such as workovers, or be contingency operations. The situation calling for a well kill will dictate the method taken.

Not all well kills are deliberate. Sometimes, the unintended buildup of fluids, either from injection of chemicals like methanol from surface, or from liquids produced from the reservoir, can be enough to kill the well, particularly gas wells, which are notoriously easy to kill.

Well control in general is an extremely expensive and dangerous operation. Extensive training, testing, proof of competence, and experience are prerequisites for planning and performing a well kill, even a seemingly simple one. Many people have died through incorrectly performed well kills.