This systematic review analyses the research results of low-achieving grade K-3 children’s numeracy competencies by investigating the research approaches used, the definitions of low achievers and the numeracy competencies reported. 18 articles, identified in ERIC, PsycINFO and Web of Science, were selected for further analysis. The results show that the main part of the studies used a fixed-strategy design, mainly reporting on children’s numeracy competencies at a group level in which the children’s numeracy competencies were summarily described and focused on difficulties and common errors. Identification of what is defined as low achiever was based on test results from both standardised and non-standardised tests, as well as teacher assessments. The predominant numeracy competencies assessed were basic facts (automatic recall 0–20) and arithmetic skills (addition and subtraction), as well as competencies related to counting. Analyses of the children’s understanding when they do not follow the typical way of learning were not found, which indicates the need for a qualitative approach to the quantitative research results in order to provide deeper understanding of children’s ways of understanding and operating with numbers.