The paper maintains that technical rules and legal certainty are linked by a symbiotic relation. Certainty amounts to predictability of the consequences of actions prescribed by legal norms conceived as technical rules (i.e.: rules prescribing conducts as means directed to achieve the agent's goals). The lack of legal certainty is a shortcoming because it impedes to use legal norms as instruments for choosing the lines of conduct more suitable to achieve our self-assigned goals. Indeed, only if the degree of certainty of a legal system is high enough, the agents can link the conduct prescribed by legal norms with consequences desirable or undesirable, using legal norms as efficient technical rules. Only in this case, it is possible to single out ex ante the «wrong» choices, avoiding those action plans that produce, more or less probably, unwanted consequences.