This chapter describes three critically important features for the planning, sustainability and implementation of strategic information systems (SIS). The literature identifies a consistent lack of success by organisations in achieving business benefits from their SIS investments and in particular the difficulties of obtaining a sustained competitive advantage over rivals. There appears to be little evidence that this record has improved as organisations increasingly rely on SIS to support their business strategy. The chapter focuses upon the need for appropriate SIS planning, the role of unique, causally ambiguous ‘isolating mechanisms’ in order to sustain SIS-derived competitive advantages and concludes by summarising the implementation factors deemed to be of real practical importance for the success of large-scale SIS projects based upon recent empirical research. The high failure rate of SIS applications in business is deemed to be largely of a managerial rather than a technical causation (Earl, 1989;Burn, 1993; Galliers et al., 1994;Barnett and Burgelman, 1996; Powell and Dent-Micallef 1997; Willcocks and Lester 1999; Watson et al., 2000). This chapter identifies and considers three components which are critical in this respect to enable an IT strategy fusion with the rest of the business (Papp, 1998).