One of the senses of the term ‘efficacy’ refers to the fact that norm-addresses actually behave as is required of them by legal norms. This sense of the term is one which is generally used within the jurisprudential discussions about whether efficacy is the essential element of the concept of law. According to legal positivism, efficacy is in some cases and in certain ways the condition of legal validity of both legal norms and legal systems. On the other hand, legal realism tends to entirely identify legal validity or reduce it to efficacy. Thus, in both jurisprudential approaches, efficacy tends to play a role in shaping their respective concepts of law. However, while the legal positivistic view does not affect the most standard sense of legal validity of the legal norm (i.e. the legal norms’ membership in the legal system), and does not add much to the explanation of the concept of law by identifying efficacy as the criterion of legal systems’ existence, the legal realistic view is faced with some serious objections regarding its explanatory adequacy.