There are several models about the development of a successful Information Systems Planning process, while very little research has focused on the contribution of the Strategic Information Systems Planning (SISP) process to firm performance. The assessment of the process is a decision problem and managers should evaluate alternatives so that it could be solved. Both executives and researchers need to be aware of how the alignment of business and IS strategies impact firm performance. The purpose of this paper is to propose a conceptual model of the determinants of the SISP process and success in Small-Medium Enterprises (SMEs) to assess the contribution of these two variables to firm performance. Understanding those phases may help IS executives concentrate their efforts on organizations’ objectives and recognize the greatest value of the planning process in their firms. This paper contributes to IS executives in Greek SMEs who do not concentrate on strategic planning during the development of IS and they focus only on the technical issues. As a result, they should understand the significance of the SISP process in order to formulate and implement IS strategy which will be aligned with business objectives and increase the success of SMEs.