Rumination and worry are common forms of perseverative cognition in children. Research has started to target perseverative cognitions in the everyday life of children, however, valid measurement instruments reliably capturing rumination and worry in children's daily life are still missing. We conducted two ambulatory assessment studies validating short scales suitable for the measurement of rumination and worry in children's daily life. Results of the first study (N = 110, 8-11 year-olds, 31 days, up to 4 daily measurements) supported a unidimensional structure of the rumination scale. Rumination was associated with negative affect (but not positive affect) on the within- and on the between-person level. On the between-person level, children who ruminated more showed poorer working memory performance. In the second study (N = 84, 8-10 year-olds, 21 days, up to 3 daily measurements), findings of Study 1 were largely replicated. Moreover, we established a unidimensional worry scale in Study 2 reliably capturing worry in children's daily life. Importantly, Study 2 showed that worry and rumination share common variance but can be differentiated in children. On the within-person level, higher levels of worry were associated with higher levels of negative affect and lower levels of positive affect. On the between-person level, worry was associated with higher levels of negative affect and lower working memory performance. Altogether, findings of both studies demonstrated that the short scales had excellent psychometric properties suggesting that they are helpful tools for the assessment of rumination and worry in children's daily life.person level, worry was associated with higher levels of negative affect and lower working memory performance. Altogether, findings of both studies demonstrated that the short scales had excellent psychometric properties suggesting that they are helpful tools for the assessment of rumination and worry in children's daily life (PsycInfo Database Record (c) 2021 APA, all rights reserved).