The purpose of this study is to investigate the behaviour of Greek students of the third class of high school on the subjects of PISA 2003. More specifically, this study examines in detail the behaviour of students in the PISA problems and compares the performance of students and the conclusion of the competition PISA 2003. Eight good students were asked to solve PISA problems. Student responses were evaluated and then conducted interviews in order to analyze the thinking of students and identify their weaknesses. Several important weaknesses of students came to light in basic subjects of Algebra, Geometry and mostly Statistics. It seemed that even most of the students applied inefficient solution strategies regardless of whether their answer agrees with common sense. It is worth mention the pleasure the students displayed during the PISA problem solving, in contrast the anxiety and fear they were occupy with, during the problem solving in class.