The study examined how strategic IS planning (SISP) success is influenced by SISP approach, SISP objectives and the context in which SISP is undertaken. Both direct relationships and fit (or contingent) relationships were researched. The main study employed multivariate analysis and structural equation modelling on data from a mail survey of the IT Directors (or equivalent) of 70 UK organisations in both the public and private sectors. A survey of 90 business executives established the absence of common methods bias. Direct relationships were supported for all of SISP approach, SISP objectives and SISP context influencing SISP success. A fit (moderation) relationship was supported of SISP objectives with SISP success, through their influence on SISP approach. Similarly, a fit (moderation) relationship was supported of SISP context with SISP success, through its impact on SISP approach. Three components of SISP context were found to be influencing SISP success (directly or through SISP approach). These were 1) the IS maturity of the organisation, 2) the business strategy orientation, and 3) the structure of the IS function. No support was found for the influence of external environment, organisation structure or culture on SISP success. Five types of SISP approach were found. These were 1) administrative, 2) IS department led, 3) technology-orientated organisational, 4) business-orientation organisational, and 5) a comprehensive approach that employed a balanced combination of other approaches. Different approaches were associated with different levels of SISP success, with the comprehensive approach being the most effective. Support was also found for a matching of different types of SISP approach with both different levels of comprehensiveness of SISP objectives and different types of SISP context. An overall model is proposed and supported through structural equation modelling of a set of relationships between all of SISP approach, SISP objectives, and SISP context influencing SISP success.