The fields of condensed-matter physics and materials science entered a new era with the discovery of high-temperature superconductors more than two decades ago. However, despite the large volume of work published on their properties, and despite their many uses, there is surprisingly little agreement in the condensed matter community regarding one of their most fundamental behaviors: the normal-superconducting phase transition. This book describes investigations of the superconducting phase transition of YBCO thin films in zero magnetic field. Simple voltage vs. current curves at temperatures close to the transition temperature show that the standard scaling analysis fails at low currents, contrary to what is widely accepted – but that there is evidence for the phase transition at higher currents. At lower currents, there are significant finite-size effects due to the thickness of the films. The crossover to from three-dimensional to two-dimensional behavior occurs even in thick films, and obscures the transition in zero field. This effect would also occur in a magnetic field, and may explain the wide range of critical exponents found in the literature.