Epac1 is a cAMP responsive exchange factor for the small G-protein Rap. It consists of a regulatory region containing a cyclic nucleotide binding (CNB) domain and a catalytic region that activates Rap. In the absence of cAMP, access of Rap to the catalytic site is blocked by the regulatory region. We analysed the conformational states of the CNB domain in the absence and in the presence of cAMP and cAMP analogues by NMR spectroscopy, resulting in the first direct insights in the activation mechanism of Epac. We prove that the CNB domain exists in equilibrium between the inactive and the active conformation, which is shifted by binding of cAMP. cAMP-binding results in conformational changes in both the ligand binding pocket, as well as the outer helical segments. We used two different cAMP antagonists that block these successive changes to elucidate the steps of this process. Highlighting the role of dynamics, the super-activator 8-pCPT-2’-O-Me-cAMP induces similar conformational changes as cAMP but causes different internal mobility. The results reveal the critical elements of the CNB domain of Epac required for activation and highlight the role of dynamics in this process.