The study analyses the mathematical competency of parents in connection with families’ social background and families’ educational processes as influencing factors on children’s competency in mathematics. Three research questions are analyzed: 1. How is the mathematical competency of parents related to families’ social background and to families’ educational processes? 2. How is the level of children’s mathematics competency related with that of their parents and with the home environment? 3. How can the development of children’s mathematical competency from 9th to 10th grade be predicted by the level of parent’s mathematics competency and by indicators of home environment? The sample used for this study consists of 73 mothers, 77 fathers, and 74 pairs of mothers and fathers whose children participated in an additional study of the Programme for International Student Assessment (PISA) in Germany. Results indicate medium correlations between families’ social background and parents’ mathematical competency. Families with higher parental competencies in mathematics tend to report more often on families’ educational processes conductive to learning. High competence of parents has a positive effect on predicting the competency level reached in mathematics by the children and on the development of mathematics competency during the course of one school year. This influence is thereby mediated by families’ educational processes.