Abstract: IT business value, the organizational performance impacts of information technology (IT), has been a fundamental research issue in MIS. While much research has been conducted, the debate on 'IT business value' continues. The purpose of this dissertation research is two-fold: first, making use of the resource-based view (RBV) of the firm and using extensions in the knowledge-based view (KBV) as theoretical foundations, this research engages in conceptual refinement and empirical validation of the IT capability construct. Then, this dissertation performs an in-depth investigation of the relationship between IT capability and firm performance and investigates agility as a mediating mechanism in determining organizational performance impacts of IT. Analyses of data from a field survey of senior business executives in 125 organizations demonstrate satisfactory construct validity and reliability of the instrument. Testing for the relationships between the three components of IT capability---Technology capability, Management capability, and Knowledge capability and organizational outcomes---provides partial support for the hypotheses. The results demonstrate a dominant effect of IT Knowledge capability and the mediating role of agility in linking IT capability to performance. However, the role of the contextual variables is not well established. An alternative competing model proposing; a hierarchical relationship among the three dimensions of IT capability is further examined within the sample context. Technology capability and Management capability are found to influence agility via IT Knowledge capability. Significant main effect of IT Knowledge capability is found on both customer agility and operational agility, while only the joint effect of IT Knowledge capability and environmental complexity is found on operational agility. However, the other two contextual variables: environmental dynamism and coordination uncertainty are not found to be moderators of the IT capability-agility relationship. Further, both aspects of agility are found to mediate the links between IT Knowledge capability and various aspects of performance. The findings of this study make significant contributions to both academic research and managerial practice.