Landfill fires are relatively common incidents that landfill operators encounter which have great impact on landfill structure and the environment. According to a U.S. Fire Administration report in 2001, an average of 8,300 landfill fires occurs each year in the United States, most of them in the spring and summer months. Subsurface spontaneous fires are considered the most dangerous and difficult to detect and extinguish among landfill fires. Few studies have been conducted on spontaneous fires in landfills. Information regarding the thermal behavior of solid waste is not available nor have measurements been made to evaluate spontaneous ignition of solid waste. The purpose of this research was to provide information concerning the initiation of spontaneous ignition incidents in landfills, and investigate the conditions favoring their occurrence. This study enabled better understanding of the self-heating process and spontaneous combustion in landfills. Effects of parameters critical to landfill operation on spontaneous combustion were determined. Spontaneous combustion occurs when materials are heated beyond the ignition temperature. Temperature rise occurs inside the landfill due to exothermic reactions which cause self-heating of the solid waste. Oxygen introduction leading to biological waste degradation and chemical oxidation is believed to be the main cause of rising solid waste temperatures to the point of ignition. A survey was distributed to landfill operators collecting information regarding spontaneous firs incidents in their landfills. Survey results raised new questions necessitating further study of subsurface fires incidents. Subsurface spontaneous fires were not restricted to any landfill geometry or type of waste (municipal, industrial, commercial, and construction and demolition). Results showed that landfill fires occur in landfills that do and do not recirculate leachate. Although new methods have been developed to detect subsurface fires, landfill operators depend primarily on visual observation of smoke or steam to detect the subsurface fires. Also, survey results indicated that excavating and covering with soil are the most widespread methods for extinguishing subsurface fires. Methane often has been suspected for initiating spontaneous subsurface firs in the landfill. However, combustible mixture of methane and oxygen requires very high temperature to ignite. In this study it was shown that spontaneous fires are initiated by solid materials with lower ignition points. Laboratory tests were conducted evaluating the effect of moisture content, oxygen concentration and leachate on spontaneous ignition of solid waste. A new procedure for testing spontaneous ignition is described based on the crossing-point method. The procedure was used to study the spontaneous combustion of solid waste and determine the auto-ignition temperature of the solid waste components and a synthesized solid waste. Correlations have been established between auto-ignition temperature, specific weight and energy content and between self-heating temperature and specific weight. Correlations indicated that compaction can help avoid spontaneous combustion in the landfill. Dense materials require higher energy to increase in temperature and limit the accessibility of oxygen. In the experimental work, moisture was found to promote both biological and chemical self-heating. Increasing moisture content lowers the solid waste permeability and absorbs more energy as it evaporates. Dissolved solids in leachate were found to promote self-heating and ignition more than distilled water. Varying oxygen concentrations indicated that heat generation occurs due to chemical oxidation even at oxygen concentration as low as 10% by volume. However, at 10% by volume oxygen, solid waste did not exhibit thermal runaway nor flammable combustion. At 0% by volume oxygen, tests results indicated occurrence of self-heating due to slow pyrolysis. A numerical one-dimensional energy model was created to simulate temperature rise in landfill for four different scenarios. Using the results from the laboratory experiment, the model estimated the heat generation in solid waste due to chemical reactions. Results from the scenario simulations indicated that moisture evaporation is the major heat sink in the landfill. The model showed that gas flow has a cooling effect due to increasing amount of evaporated water and can control the temperature inside the landfill. The model showed that a temperature higher than the biological limit can be maintained in the landfill without initiating spontaneous fire.