The efficacy of a firm depends on its comparative advantages and the suitable decisions in Human Resources are one of them. In quite huge firms, human resources managers have among their main goals the election and right promotion of the firm workers, attending their competences. Given that uncertainty is present in any valuation of the candidates' competences and qualities, we propose appropriate methods for their treatment, based on Fuzzy Sets. In particular, we use interval-valued fuzzy sets for valuating; OWA techniques, Ordered Weighted Averaging operators, for aggregating; and similarity techniques, Hamming' distance and Matching Level Index, for comparing the competences of the candidates. All these techniques are applied in the StaffDesigner oftware that we have developed. The last section of the paper is dedicated to the study and valuation of a personnel promotion in a private firm case.