This chapter addresses the teaching and assessment of mathematical activities for children with Intellectual Developmental Disorder (IDD), arguing that mathematics should be linked to everyday life, especially to activities of daily living. It starts from the concept of numeration, understood as mastery of the number system and basic operations applied to practical situations. Pedagogical principles for effective teaching are presented: identifying the student’s prior knowledge, organizing small teaching units, gradually increasing complexity, ensuring prerequisites, promoting observation and imitation of models, using concrete materials, and ensuring generalization to real-world contexts. The chapter describes assessment protocols, such as the one developed by Rossit, based on stimulus equivalence, and the PRAHM, developed by Costa and colleagues, focused on counting, pre-arithmetic, and recognition of geometric figures. Activities involving counting principles, use of the abacus, teaching area and volume with concrete sequences, and introduction to algebra are also presented. It concludes that structured interventions, with visual support and intentional mediation, favor the acquisition, fluency, maintenance, and generalization of mathematical skills.