The success of strategic information systems planning (SISP) is of paramount importance to academics as well as practitioners. SISP is a management process that enables organizations to successfully harness the power of current- and next-generation information systems (IS) applications to fulfill their business goals. Hence, by capturing the major constructs that can influence SISP success, the SISP theory has made significant contribution to research and practice. However, the literature shows that, despite this important contribution, the SISP theory has only been used to a limited extent. Aiming to address this scarcity of SISP theory utilization, this study developed a research model that integrates four constructs derived in large part from the SISP theory. The four constructs are: (a) the extent of top management support, (b) the usefulness of the information systems plan, (c) the degree of Information technology infrastructure flexibility, and (d) the degree of SISP success. Based on the contingency theory, the usefulness of the information systems plan (ISP) and the IT infrastructure flexibility (ITIF) were examined as two conditions that can mediate the effects of top management support on SISP success. Data analysis was conducted using Partial Least Squares, which is a type of structural equation modeling that accommodates small sample sizes. From a sample of 57 information systems executives from US organizations, support was found for the hypothesis that top management support has a direct, positive, and significant effect on SISP success. Also supported are the hypotheses that top management support has a significant effect on the usefulness of the ISP, which in turn has a similar effect on SISP success. By contrast, support was not found for the hypotheses that top management support has a significant effect on ITIF, which in turn is significantly related to SISP success. Top management support was shown to influence the degree of SISP success both directly and indirectly through the usefulness of the organizations' ISP, but not through their IT infrastructure flexibility (ITIF). Unlike ITIF, the usefulness of the ISP was confirmed to be a significant mediator of the effects of top management support on SISP success. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]