In 1951, Piaget and Inhelder contended that young children’s understanding of probability does not begin to develop until after the age of 7. This seminal work reduced expectations of young children’s capacity to engage in probabilistic thinking. However, recent research studies have questioned and begun to dispute the contentions of Piaget and Inhelder, revealing young children’s potential to engage with complex probabilistic ideas (e.g., Kingston & Twohill, 2022; Nikiforidou, 2018, 2019). Nonetheless, probability remains absent from many international primary mathematics curricula at the younger class levels. Learning trajectories/pathways have been increasingly drawn upon as applicable theorisations of children's mathematical thinking (cf. Clarke & Roche, 2009; Clements & Sarama, 2014). Within the study described in this thesis, frameworks of growth points that describe the development of young children’s probabilistic thinking were constructed, along with tasks designed to assess children’s thinking along these growth points. A design-based research approach was adopted which enabled the frameworks and tasks to be implemented, evaluated, and modified across three iterative phases, as proposed by Anderson and Shattuck (2012). 72 children aged between 4 and 8 were interviewed from five schools. One-to-one task-based interviews were utilised to gather qualitative data relating to the children’s probabilistic thinking. Each phase had a specific focus; Phase 1 was concerned with the internal structure of the designs, Phase 2 focused on their application, and Phase 3 examined their potential effects and impact (McKenney & Reeves, 2014). The findings of this study indicate that the established frameworks of growth points and the corresponding tasks can be utilised to describe the development of young children’s probabilistic thinking. The results also suggest that young children can engage in rich probabilistic thinking and that informal quantitative thinking plays a key role. Across all phases and constructs, the majority of the children’s thinking was dominated by quantitative thinking but due to their limited experience of part-whole reasoning, these ideas were often expressed in informal ways. This finding was consistent across all class levels, with over 83% of the children’s thinking being ranked as reflecting Growth Point 3 (informal quantitative thinking) on each of the three frameworks during the final phase of the study. The study’s findings highlight the need for an earlier inclusion of probability in the primary mathematics curriculum and call for further research to identify effective approaches for teaching probability to young children. The detailed insights gained into the children’s thinking through the use of task-based interviews also advocates for their inclusion as a recommended assessment tool in the primary mathematics curriculum.