In this study, experiments and models are developed to understand the thermal runaway behavior of lithium-ion pouch cells. A battery system typically consists of multiple cells arranged in combinations of series and parallel electrical connections. For the purposes of thermal analysis, the geometric configuration must be clearly specified to determine the effects of heat transfer between cells and from cells to the ambient or to a cooling fluid. The possibility of thermal runaway and the rate of heat release are affected by the thermal packaging of the cells and the cell thermophysical and kinetic parameters. The thermal conductivity of the cells is determined by using experimentally measured temperatures and a forward model within a Bayesian parameter estimation framework. Two models of cell thermal runaway are developed. The simpler model, considers the cell to be lumped. The more complex model uses a 1D finite difference method to model the cell thermal runaway evolution. The methodology discussed in this study is a starting point for characterizing how thermal runaway propagates in arrays of multiple cells. This methodology can be applied to different battery types used in hazard analysis of various battery systems.