This study explores difficulties that prospective elementary mathematics teachers have with the concepts of ratio and proportion, mainly when they are engaged in solving problems using algorithm procedures. These difficulties can be traced back to earlier experiences when they were students of junior and high school. The reflection on these difficulties by student teachers, comparing to informal ways of solving the problems is a fundamental step of the pre-service programme in which they are involved. In this communication I also present and discuss an attempt to promote development of prospective teachers' own knowledge of ratio and proportion as well as their awareness of the pupils' difficulties on this subject. (Contains 2 tables.) [The work described in this paper is part of a project named "Teachers and New Competencies on Primary Mathematics", funded by Fundacao para a Ciencia e Tecnologia, Portugal. For complete proceedings, see ED500858.]