Conceptual understanding, particularly geometric conceptual understanding, is central to mathematics education. The development of conceptual understanding is a complex, non-linear process that begins in childhood and continues throughout life. The present study examines the conceptual understanding of the term “quadrilateral” by 306 participants including pre-service teachers and primary school children from grades 1 to 4. Participants were shown 18 shapes, some of which were quadrilaterals and others non-quadrilaterals, and were asked to identify the quadrilaterals. The results indicate that neither primary school children nor pre-service teachers have a sufficient understanding of the concept. While prototypical figures tended to be better recognized, the concave quadrilateral appeared to cause particular difficulties.