This study aims to analyze how the Portuguese National Union of Psychologists (Sindicato Nacional dos Psicólogos) has been influenced in organizational terms by the factors that have brought about the principal changes in its history. The central aim of the study is to provide a historical outline of the institution's history, focused on its principal moments of change. By doing so, the study accomplishes it's theoretical contribute by analyzing the history of the institution from an organizational phenomenon's point of view - change. Inherent to completing this central aim was the need to identify which organizational lifecycle phase the SNP is currently in, as per the Hisrich and Peters model (2002). This case study is, primarily, descriptive in nature. Based on data collected from oral interviews, archives and other documentation, the study concludes that the history of the SNP has been influenced by seven key moments, and that in terms of the above-mentioned model, the institution is currently in a rebirth phase, or possibly even in a new growth phase. [ABSTRACT FROM AUTHOR]