Besser, NadineNadineBesser0000-0002-1250-1457Linberg, AnjaAnjaLinbergDornheim, DorotheaDorotheaDornheim0000-0001-5161-9207Weinert, SabineSabineWeinert0000-0002-8341-9821Roßbach, Hans-GüntherHans-GüntherRoßbachLehrl, SimoneSimoneLehrl0000-0002-2720-122X2025-04-112025-04-1120251873-77060885-2006https://fis.uni-bamberg.de/handle/uniba/107542The first years of life are crucial for children's concurrent and subsequent development in various domains. Given the recognized importance of high-quality adult-child interactions for promoting young children's development during this vulnerable phase, supporting such interactions is essential. This study examines the impact of a 9-week domain-specific mathematical interaction training for early childhood education and care (ECEC) teachers on children's development of numeracy skills and mathematical language from age two to four years (N = 408 children). Specifically, it investigates whether the children in groups with teachers trained math-specific, would achieve better outcomes on numeracy skills and mathematical language than a control group who followed their usual program, and, to test for the math-specific effect, to a control-intervention group who received a general interaction training. Moreover, effects on math outcomes were contrasted to general language outcomes to cross-check the training's specific impact. Results showed significant improvements in numeracy skills, in mathematical and general language over time. The math-specific intervention had particularly strong and lasting effects on numeracy and mathematical language, while the general intervention specifically improved general language compared to the control group. The two intervention groups differed significantly in their math and language skills at posttest, but these differences were reduced at follow-up testing. These findings underscore the effectiveness of targeted professional development (PD) training in fostering children's early numeracy skills and mathematical language. By systematically varying intervention content and controlling for environmental factors, this study provides valuable insights into the benefits of math-specific training for ECEC teachers.engNumeracy skillsMathematical languageToddlersHigh-quality interactionsDomain-specificIntervention370article10.1016/j.ecresq.2025.02.004