This research is based on the analysis of answers given by undergraduate physics students when confronted with situations related to the Special Theory of Relativity. Historical and epistemological studies indicate the importance of principles for the construction of physical theories. There is a consensual acknowledgement of the fundamental role played by the Principle of Relativity in the case of Special Relativity. The study investigated the extent to which that principle was employed by the students when dealing with a diversity of phenomena. The research data were obtained through clinical interviews and supported two main conclusions: (a) the Principle of Relativity is not used as a heuristic tool in the students' answers; (b) the situations presented were not regarded as 'problematic' by the students, who did not feel the need to use interpretative structures other than those of classical mechanics and common sense.