The knowledge of the actual magnitude and main features of tourism flows in a given destination is an essential prerequisite for the evaluation of tourism impacts and externalities. Indeed, many pleasure trips are often characterized by the visit to more than a single destination. Although the topic is well-documented in literature, the empirical results are limited to a few pioneering studies. The lack may be attributable to the failure of tourism organizations to collect data on multi-destination trip behaviour. This can be seen, for example, in the system of European statistics on tourism (according to the Council Directive 95/57 EC), where information on the average number of visited destinations within a single trip is not provided. This paper aims at discussing the main implications of multi-destination trips, both on tourism statistics and on destination management. It also proposes to describe the research design and main preliminary results of a survey on incoming tourism in Sicily. Some remarks dealing with the positive and negative impacts of unobserved tourism and of multi-destination trips are reported at the end.