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In constitutional theory, the requirement of necessity is an integral part of a wider proportionality assessment in the limitation of constitutional rights. It fulfils a function of sorting out measures that restrict rights beyond what is required to fulfil the intended purpose. Within data protection, the requirement varies in strictness and interpretation—from ‘ordinary’ necessity to ‘strict necessity’. Recently, the European Court of Justice (ECJ) has introduced what appears to be an even stricter requirement of ‘absolute necessity’ relating to the processing of biometric information under the EU Law Enforcement Directive (LED). In practice, however, the implications of those respective levels of strictness tends to vary, from a strict ‘least restrictive means’ test, to an analysis of whether a measure is necessary for a more effective or a more efficient fulfilment of the intended purpose. In this contribution the principle of necessity as applied by the ECJ is analysed as it pertains to the LED and the Charter, more specifically in the context of implementing AI supported analysis of biometric data. The gradual development of the interpretation of necessity is traced in the data protection case law of the ECJ. The study shows the increased emphasis placed on proportionality over time, highlighting both strengths and potential weaknesses of the requirement in relation to the use of AI supported decision-making in the law enforcement context.