The core critical thinking skills, identified in "The Delphi Report" as essential elements for workplace and educational success, are targeted in a standardized 35 item multiple-choice assessment tool entitled the "Test of Everyday Reasoning (TER)" which is designed to provide a representation of a person's overall critical thinking ability. In this paper, the reliability of the Greek version of the "Test of Everyday Reasoning (TER)" is investigated. In the research participated 350 Greek students. Reliability analysis was conducted for the overall sample of the research and the subgroups: (a) young students (11-13 years old) and university students, (b) male and female. Data showed that the Greek TER was a reliable instrument measuring critical thinking for the above mentioned groups. (Contains 2 footnotes.)