Corporate diversification continues to be an important phenomenon in the modern business world. More than thirty years of research on diversification suggests that the degree of relatedness among a firm's business units is a factor that can affect firm performance, but the true effect of diversification relatedness on firm performance is still inconclusive. The purpose of this dissertation is to shed more light on this inconclusive association. However, attention is focused on the performance implications of individual diversification actions (e.g., acquisitions and joint ventures) rather than on the overall performance of firms with different levels of diversification. A non-experimental, longitudinal analysis of secondary data was conducted on over 450 unique acquisitions and on more than 210 joint ventures. Results suggest that even when individual diversification actions rather than entire business portfolios are examined, an inverted curvilinear association between diversification relatedness and performance is likely to emerge. This pattern is observed in both acquisitions and joint ventures. However, the association between diversification relatedness and performance in acquisitions is moderated by the level of industry adversity, though factors such as corporate coherence and heterogeneous experience do not moderate the association between diversification relatedness and performance. This study augments the body of knowledge …