Children's ability to monitor subjective feelings of uncertainty (i.e., engage in uncertainty monitoring) is a central metacognitive skill. The study examines the development of uncertainty monitoring as well as its relations with vocabulary and executive function development in children (N = 137, 52% female) from predominately White and Latinx/Hispanic backgrounds when they were 4-6 years old and enrolled in a Head Start preschool and kindergarten between 2018 and 2019. The study found that children's uncertainty monitoring improved during the kindergarten year. Children's executive function and vocabulary in preschool and vocabulary growth from preschool to kindergarten predicted uncertainty monitoring at the end of kindergarten, which sheds new light on potential mechanisms supporting children's metacognitive development. [This is the online version of an article published in "Child Development." For the final published version of this article, see EJ1329121.]