This document defines concepts and identifies relevant issues related to stability in microgrids. It proposes a definition and a classification of microgrid stability, taking into account pertinent microgrid features such as voltage-frequency dependency, unbalancing, low inertia, and generation intermittency. The modeling of microgrid components such as generators, converters, distribution lines, loads, and distributed energy resources for stability analysis is discussed in detail. Analysis techniques and tools relevant to microgrid stability are also reviewed, as well as various examples highlighting some of the stability classes defined in this report.