In seven experiments this thesis explored the processing responsible for children’s understanding of uncertainty. Through distinguishing between types of evidence and types of uncertainty a new framework was created in which to investigate the relationships between children’s varying metacognitive abilities. Experiments 1 to 3 focussed on 5-to-7-year-olds’ behavioural awareness of uncertainty, exploring the basis of children’s confidence judgements and their influence on strategic behaviour. Children demonstrated a dissociation between these two behaviours suggesting younger children’s metacognitive abilities are based on a two-system process. Experiments 4 to 6 investigated children’s behavioural sensitivity to uncertainty, exploring the relationship between children’s ease of imagining and their decision making. An interesting difference in response latencies was demonstrated across types of uncertainty, suggesting children’s strategic behaviours are based on the ease with which an answer comes to mind. In Experiment 7, 4-to-8-year-olds’ alternative sensitivity was explored, examining children’s appreciation of possibilities through a novel eye-tracking paradigm. Children demonstrated an acknowledgment of the multiple possibilities associated with partial ignorance suggesting a possible meta-representational basis to children’s metacognitive abilities. Taken together these findings offer new insights into the development of children’s metacognition and the processing systems behind their uncertainty understanding.