Managers at all organisational levels make more or less consciously various decisions day after day. In non-trivial situations the decision-making process is often methodically supported. Practical experiences show that even the choice of appropriate management tools for supporting decision-making processes is a well known decision-making problem itself. Within the presented thesis the author attends to this problem and develops a selection model for supporting the decision-making process. Initial points are the decision-oriented business management as appropriate perspective and constructivist positioning within the philosophy of science. Relevant chapters of decision theory including decision-making processes and complexity of decisions are discussed as well as tools which are substantial for this thesis and existing concepts. Core of the thesis is a developed phase model for complexity-adequate support of the tool choice. The model is concretized and analysed by means of two case studies. One case study investigates the decision-making process of a melting crucible investment within the steel industry; the other one examines investments of E&P companies in the petroleum industry.