Many different quality approaches are available in the software industry. Some of the approaches, such as ISO 9001 are not software specific, i.e. they define general requirements for an organization and they can be used at any company. Others, such as Automotive SPICE have been derived from a software-specific approach and can be used for improving particular (in this case automotive) processes. Some are created to improve development processes (e.g. CMMI for Development), others focus on services (e.g. CMMI for Services), and again others are related to particular processes such as software testing (e.g. TMMi) or resource management (e.g. People CMM). 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 compliance to a number of standards. First of all, it has to be decided which approaches have the 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 on 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. Accordingly, we addressed the following research question: Does the PBU framework provide a solution of sufficient quality for current problems of simultaneous usage of multiple quality approaches? In order to recognize a solution of sufficient quality we identified criteria for multi-model solutions based on current problems and initiatives. This is called MSPI criteria. In order to determine if the mapping of quality approaches to a process is possible we analyzed elements of quality approaches, elements of processes and the mapping of quality approach elements to process elements. The findings of the analysis lead us to design a multi-model solution, the PBU framework. In order to show the feasibility of the results, we performed a case study reflecting how the PBU framework can be used in practice for developing a unified (peer review) process which conforms to three different quality approaches. Finally, we assessed the PBU framework against MSPI criteria. With the case study and the assessment of the PBU framework, we answered the research question and provided a proof of concept of the research.