The objective of this study is to examine the tour operators' influence on the long term sustainability of destinations. The tour operators' activities are considered in detail in order to verify the hypothesis that the tour operator is one of the major influences for the nature of tourism demand. It is suggested that they are also instrumental in determining market trends and may affect the demand levels for the destinations. To verify the degree of influence by the mass and specialist tour operator, of varying sizes, their marketing strategies and operational tactics need to be examined critically in order to determine their hypothetical level of influencing the snstainability of a tourism destination. Tourism destinations in a long term successful sustainable tourism industry depend on the maintenance and sustenance of the 'right' level of demand. The tourism supply is mutually affected by demand characteristics. Sustainable tourism is dependent on the effective co-operation of all the stakeholders in the industry, for example, suppliers, intermediaries, public sector and consumers. This article reviews the trends of the UK tour operators; it examines the literature on sustainable tourism, reports the field research findings and discusses the relationship and effects of tour operators upon the sustainability of tourism destinations.