There has been considerable analysis of the potential for spontaneous combustion of individual kegs of self-heating materials, such as calcium hypochlorite. However, when arrays of such kegs are shipped in a single closed container, interaction effects become significant, and the critical value of the external temperature for self-ignition is significantly lowered. In this book, we consider the extent to which simple models, which are amenable to analytic treatment, can mimic this behaviour: thus we extend the well-known Frank-Kamenetskii approximation to a 2D model with infinite height to deal with a cylinder surrounded by air and an external shell, a cylinder surrounded by an annular region of same reactivity and so on. These results are found to be consistent with numerical solutions and experimental data. The model is further extended to 3D to incorporate the effect of finite height as in reality. The book should shed some light on the research in Spontaneous Combustions for physicists and provide inspirations in Fire Hazard Prevention for future ship engineers.".