The objective of my dissertation is to understand the process of flame spread over solid fuels in the presence of an opposing flow. To this end numerical solution of the governing equations is used to prioritize the involved physics in order of impact on the flame spread rate. The results of this prioritization has lead to a Simplified Theory which has proven to be an accurate model of the flame spread rate and flame structure in one regime of flame spread. Application of the numerical model to other regimes of flame spread has lead to discovery and understanding of new phenomenon.