Plasmoids everywhere: ideal tearing, the transition to fast reconnection, and solar activity.

We discuss the role of generalized ``ideal" tearing (IT) as a possible trigger mechanism for magnetic reconnection to understand energetic phenomena in the solar atmosphere. We begin with a pedagogical introduction to the IT concept, how it stems from the classical analysis of the tearing instability, what is meant by plasmoids, and the connections of IT to the plasmoid instability and Sweet Parker current sheets. We then proceed to analyze how the IT concept extends to equilibria with flows, small scale kinetic effects, different current structures and different magnetic field topology configurations. Finally we discuss the relationship of reconnection triggering to nonlinear cascades and turbulent evolution, and how different situations may arise depending on scale, boundary conditions, and time-history, from coronal heating via nanoflares, to solar flares and coronal mass ejections. Issues of local topology, dimensionality, anisotropy will also be discussed.