Problems of the class NP it's almost all problems solved on the computer. Therefore, this is extremely important and actually to research the properties of such problems and to construct their mathematical models, which allows in a number of cases to improve the solution algorithms or propose new ones. In studying the problems of the class NP we focused on the NP-complete problems, the researching their properties and constructing models. We construct a mathematical model of constructive combinatorial problems, clarify the concept of a class of problems solved by a non-deterministic Turing machine and define the concept of the problem without foresight, investigate the set-theoretic properties of extreme combinatorial problems. We offer the polynomial-time algorithm for the maxumum independent set problem based on a hypotheses. Also, we find a criterion for Hamiltonicity of a graph and consider some covering problems. This book should be especially useful to professionals in computer sience.