We study the effect of twin boundaries (TBs) on the critical current density of YBa 2 Cu 3 O (YBCO) films and nanocomposites grown on different substrates. Varying both the direction of the current and magnetic field, we show that the TB orientation is a crucial parameter to consider in the optimization of J c for particular applications. A quantitative and detailed analysis of the role of TBs on vortex dynamics has allowed us to infer that extended TB planes in pristine YBCO films can reduce J c by 60% at low temperatures due to vortex channeling effects or increase it by a 98% at high temperatures due to directional vortex pinning. Moreover, we demonstrate that TB length and distribution can be strongly modified in YBCO nanocomposites. We observe that TB planes with no vertical coherence are still effective for vortex pinning while are not to create channels for easy vortex flow.