In this book, solutions of nonlinear parabolic differential equations are investigated. More specifically, existence and non-existence theorems of solutions are presented. The first part involves the related results for the Cauchy problem for a set of nonlinear parabolic differential equations with critical exponent. Then the necessary and sufficient conditions are determined for the non-existence results of solutions for the corresponding Dirichlet and mixed boundary value problems. The proofs make use of the dilation and comparison arguments. Moreover, to generalize these results, proofs are given in n-dimension and for general type of domains. Finally, numerical simulations are also provided to verify the theoretical analysis