One purpose of the present paper is to elaborate 4 general advantages of the mixed methods approach. Another purpose is to propose a 5-phase evaluation design, and to demonstrate its usefulness for mixed methods research. The account is limited to research on groups in need of treatment, i.e., vulnerable groups, and the advantages of mixed methods are illustrated by the help of the 5-phase evaluation design. The basic idea is that the total set of relevant attributes and changes for such a vulnerable group should be taken into consideration in all phases, and that the mixed methods approach will provide an optimal treatment, will give a more complete description and understanding of the treatment effects, and will facilitate generalization to professional work. (Contains 1 figure.)