Good performance in Mathematics remain a challenge for most Namibian learners. Year after year many learners at Grade 10 and Grade 12 level perform very poorly in this subject, which limits their chances of tertiary education. One of the reasons for poor performance in Mathematics pointed out in research was poor development of numbers sense in the Junior Primary school years. The purpose of this study was to find out how teachers in the Junior Primary understood number sense development, its importance for doing mathematics and how teachers' undertanding of this key aspect influence the way they teach number sense development.