Let's Count is a mathematics professional learning programme for preschool educators in Australia, managed by a prominent non-government organisation and sponsored by industry. It has been implemented in both face-to-face and online modes over 2013/14. Let's Count is based on the constructs that all young children are powerful mathematicians and that children should be given opportunities to access mathematical ideas through play and interactions with peers and adults, including family members. In this paper, we report on an evaluation of the impact of Let's Count on the educators' beliefs, expectations, aspirations and practices around preschool mathematics education. The evaluation utilises a mixed-methods approach in which educators have responded to brief written surveys as well as email and telephone interviews. The main findings from this research are demonstrable shifts in educators' beliefs and attitudes about, expectations of and aspirations for the mathematical learning of young children. These shifts have resulted in rich mathematical learning opportunities for children and conversations with both children and adult family members in preschool settings. Findings from this research suggest that Let's Count, in both of its modes, has had a positive impact on the children's mathematical learning as well as the educators' beliefs and practices. (author abstract)