The book discusses Functional separable method (FSM) and its applications. In this book we have examined the nonstationary heat equation with a nonlinear source, the nonstationary heat equation with a logarithmic source, the nonlinear Klein-Gordan equation and the Kolmogorov-Petrovsky-Piskunov (KPP) equation. Also, we have analysed mathematical modelling of moisture content in unsaturated heterogeneous soil and its solution by using Functional separable method.