This chapter discusses the teaching and assessment of mathematics for children with Autism Spectrum Disorder (ASD), questioning the myth that students with ASD, without intellectual disabilities, have a natural aptitude for the subject. Studies show that many present difficulties, especially in problem-solving, working memory, verbal comprehension, and math anxiety. Widely used assessment instruments are analyzed, such as the Wechsler Individual Achievement Test, Third Edition, the Test of Early Mathematics Ability–Third Edition, and the PRAHM protocol, highlighting the importance of identifying initial repertoires, strengths, and specific deficits. Research indicates relatively better performance in calculations than in verbal problems, suggesting the impact of communicative difficulties. The chapter proposes activities organized into teaching units, advocating for teaching in small steps, the use of concrete and digital materials, gradual cues, and intentional planning to promote autonomy. The chapter concludes that assessing initial repertoire is the first step towards effective teaching, since there is no single formula, but rather evidence-based principles that guide the teacher in the face of the diversity present in ASD.