This paper demonstrates that Sequential Lateral Solidification (SLS) of Si can be carried out on films as thin as – and potentially much thinner than – 250 Å. When compared to thicker Si films, however, the SLS-processed ultra-thin films contain more twins, and successful processing requires irradiation within a narrower laser energy density range and a smaller per-pulse translation distance. The physical interpretation of these findings is formulated by analyzing the details of the microstructures observed in single-pulse-irradiation-induced Controlled Super-Lateral Growth (C-SLG) experiments. SEM and TEM analyses reveal complicated microstructural details that we interpret as originating from breakdown of epitaxial growth during lateral solidification, an effect that is detrimental to the SLS process. Based on considerations of far-from- equilibrium solidification behavior of Si, it is argued that undercooling of the solidification interface below a threshold value at which solidification no longer proceeds epitaxially – arising from reduction in interfacial recalescence during lateral solidification of ultra-thin Si films, relative to that of thicker films – is responsible for the breakdown. Based on this model, we discuss how external parameters may be adjusted so as to permit optimal crystallization of ultra-thin Si films using SLS.