A number of differences among quality approaches exist and there can be various situations in which the usage of multiple approaches is required, e.g. to strengthen a particular process with multiple quality approaches or to reach certification of the compliance to a number of standards. First of all it has to be decided which approaches have potential for the organization. In many cases one approach does not contain enough information for process implementation. Consequently, the organization may need to use several approaches and the decision has to be made how the chosen approaches can be used simultaneously. This area is called Multi-model Software Process Improvement (MSPI). The simultaneous usage of multiple quality approaches is called the multi-model problem. In this dissertation we propose a solution for the multi-model problem which we call the Process Based Unification (PBU) framework. The PBU framework consists of the PBU concept, a PBU process and the PBU result. We call PBU concept the mapping of quality approaches to a unified process. The PBU concept is operationalized by a PBU process. The PBU result includes the resulting unified process and the mapping of quality approaches to the unified process.