The researchers in this study investigated the impact of mathematics-focused parental involvement on Kindergarten to Grade 8 children and parents as well as factors prompting that impact. Qualitative analysis consisting of parent, child, and teacher interviews and 3-year quantitative testing showed significant improvements in students' mathematics understanding and achievement. Moreover parents and teachers improved on measures of content knowledge. We hypothesize that improved parent content and pedagogical knowledge and improved parent-child interactions around mathematics stimulate children to learn at school. Furthermore, we found aspects of parent knowledge and dispositions gained to be analogous to teacher MKT; we termed these aspects "Mathematical Knowledge for Parental Involvement". Such aspects include content knowledge, valuing students' own strategies, and listening to students' explanations and may boost student achievement in mathematics.