This paper seeks to demonstrate that, when a public entity determines the requirements of a public bidding process, it affects the freedom of bidders and the efficient destination of public funds. Therefore, the determination of such requirements, in order to be legitimate and efficient, must be proportional. As shown in this paper, the proportionality principle, a general notion about the relation between means and ends, applies to the determination of require- ments in public bidding process. Furthermore, this paper seeks to explore the way in which this principle can be applied.