While the ethics and politics of nudging have received a great deal of attention, the legality of non-coercive interventions aimed at changing human behavior has received less attention. In this contribution, I examine firstly which limits, if any, the principle of proportionality (“Verhältnismäßigkeitsgrundsatz”) as applied by the German Federal Constitutional Court imposes on governmental “choice architects”. While nudges as such generally do not interfere with fundamental rights, including the very broad “right of personal development” of art. 2(1) Basic Law for the Federal Republic of Germany, their implementation will often interfere with the fundamental rights of citizens other than the decision makers and therefore trigger constitutional scrutiny. Since paternalistic motives are – in principle – not accepted as legitimate ends that may justify an interference with fundamental rights, nudges solely intended to protect the decision maker from self-harm may not pass the proportionality test. Secondly, I examine whether the necessity prong of the proportionality principle may force the legislator to choose a non-coercive nudge over a more traditional coercive measure. Given the empirical evidence of the limited effectiveness of nudges in changing behaviour compared to traditional regulatory means, I come to the conclusion that the proportionality principle does not compel the use of nudges.