This chapter analyses legal status and certainty of Portable Documents A1 that certify the applicable legislation (hereinafter “PDs A1”). Has this important figure of law been transformed into a useful tool that ordinary employers, employees and third parties may rely upon? Not yet because, seen form Member States’ perspective, this would mean to loose another bit of their sovereignty. An important and inevitable step that must be undertake if we want an efficient internal market. Effective coordination of Member States’ social security law requires the unification of the authorities that issue PDs A1 and a change from a certificate of affiliation to a decision on affiliation to a specific social security scheme.