The way teachers understand mathematics strongly influences their teaching and what their pupils learn. Using Vinner's model of acquisition of mathematical concepts with its distinction of concept image and concept definition as a framework, we analyze concept images, difficulties, and errors related to the concept of altitude of a triangle exhibited by 190 preservice primary teachers in a written test. We describe the influence of two variables on the preservice teachers' performances: (a) the presence of a formal definition and (b) previous classroom activities that dealt with the concept of altitude. We categorize and analyze some common errors and identify the concept images that may lead to those errors. Finally, we present some implications of our results for teacher education.