The contents of this book deals with the diffusion and the efficiency of ISO 9000. This study (done in Portugal) has been carried under the scope of the management literature review. A model of analysis was constructed under the umbrella of three management theories – institutional, resources based view (RBV) and contingent. The empirical studies, qualitative and quantitative, were achieved in two Portuguese ISO certified sectors of activity and in one not certified sector. Qualitative analysis was based on interviews in the organizations/sector. Quantitative elements were got from a database, enabling a statistical analysis, a correlation analysis and an econometric model. Findings relate: the diffusion of ISO 9000, from 2010, seems to fit in the forecast models of the specialized literature; as to efficiency, Portugal is far from the intensity of certification verified in Spain. Qualitative data reveals that quality is a real must to be in the market and quantitative information confirmed the literature as to the size of the organizations exhibing an ISO 9001. Finally a theoretical holistic approach was defined based on the above considered management theories.