Software Process Improvements includes changing the way people work and use software, and CMM is a model for the description and assessment of maturity in a company's work with software develpoment with focus on the development processes. Based on an interpretive study in two deparments in Norway's largest telecommunications company using two different approaches to CMM-based assessments, I define prerequisites for success for using the methods, and of software process improvement in an organization like the one at hand. A quantitative approach could be used when CMM concepts are institutionalized in the organization and the goal of the assessment is CMM compliance, to measure Software Process Improvement progress, or evaluate CMM appropriateness in the organization. A qualitative approach should be used when the goal is to get in-depth information about good and bad practice and to locate improvement areas, and when the appraisal team has the necessary skills. The prerequisites that need to ge in place to ensure success of CMM-based Software Process Improvement are commitment, management, participation, and risk management.