This chapter looks at transactional security, which is a condition precedent to a viable automated marketplace allowing for the conclusion of binding contracts. It identifies the European Union legal measures that are relevant to transactional security, and examines them in search for the aforementioned legal obligation. In any event, the electronic signatures directive evinced a strong emphasis on online transactional security and a willingness to encourage the employment of effective security measures in electronic transactions. A legal obligation to ensure transactional security fits better within a measure intended to take a more direct approach towards online security, thereby addressing relevant obligations to the actors who can employ the various security-enhancing solutions. A liability rule is all the more important in the light of past experience showing that damage-causing poor security practices employed by trust service providers may go unnoticed by independent audits.