The study of sustainable tourism from the perspective of the consumer is becoming central to tourism managers, who are now clearly aware of the importance of keeping long lasting relationships with their customers on the basis of responsible proposals. In this context, the present work analyzes the process of loyalty formation in its two components (attitudinal and behavioral). The analysis starts from a series of preceding variables (perceived value, service quality, tourist satisfaction and commitment) accompanied by a set of modulating variables (tourist load capacity, search for variety, and sociodemographic features of the tourist). A literature review on the nature of loyalty allowed not only proposing a causal model for estimating it in the case of tourism, but also formulating a series of hypotheses that shall be contrasted by further research. The study of the variables here considered provides tourism companies with better knowledge on the process that leads to tourist loyalty. This, in turn, allows them to focus their marketing efforts on keeping long lasting relations with their customers, thus contributing to the development of sustainable tourism.