As we all know, teaching mathematics so that it is understood by students is one of the fundamental problems in mathematics. It is approached both from the point of view of communication in mathematical language in the classroom, and from the point of view of pedagogy and general didactics as an integrative means of formal education. Both perspectives are complex. The objective of this article is a concise analysis of a fundamental concept, namely the mathematics knowledge of the teacher (MKT), both for the teaching of mathematics for the primary cycle and for the way in which teachers are trained. for the primary cycle. (MKT) is analyzed in terms of the curriculum of the subject study program "Mathematics and exploration of the environment" for the first three classes of primary school. The importance of (MKT) to "decipher" the complexity of teaching and learning mathematics in primary school in the context of theories of integration of sciences is a current challenge for mathematics education and also for pedagogy.