In an attempt to clarify the origin of the large critical current densities Jc observed in Laser Ablated and Sputtered YBa2Cu3O7-δ (YBCO) thin films, we make a systematic study of the low temperature Jc in samples carefully analysed using STM and AFM. Jc (B) is determined from torque-magnetometry performed in ring-patterned thin films. Epitaxial YBCO films nucleate in e-axis oriented single-crystalline islands with sizes ranging between 200 and 700 nm. We show that Jc can be mainly explained by vortex pinning loealised in the island boundaries.@en