Wildfires are complex phenomenon. To understand their spread mechanisms, physics based wildfire models are developed here. Discontinuous fire spread, i.e., spotting, is studied through a review of historical wind-driven and post-earthquake fires. Transport of various shapes of combusting firebrands is modeled using momentum balance on the firebrand. Firebrand trajectories are simulated in velocity and thermal fields generated by the FIRETEC wildfire model. Contiguous fire spread is modeled based on energy conservation and detailed heat transfer mechanisms. Predictions of contiguous fire spread rates are compared to laboratory experiments and prescribed fires.