In part I of this article, the current understanding and experimental observations of the so-called first breakdown (BD) phenomena are reviewed and summarized with a focus on BD statistics and voltage/field acceleration models because of their critical importance to reliability projection. Experimental BD data over a wide range of dielectric materials are reviewed together in a common framework. A thorough examination of various analytic BD models with key features is provided in comparison with experimental observations. In addition, we highlight advanced numerical BD models, which bring out more detailed aspects of the BD process. This state-of-the-art review can provide researchers and engineers with a coherent, global understanding to help continue their research work in this exciting field of dielectric BD.