Several tools for assessing mindfulness in adults, with good psychometric properties, are available. However, for children, these adult questionnaires are not validated, and items might be worded too abstract or complex. Nevertheless, the development of mindfulness measures for children is necessary in order to understand the role of mindfulness skills and to identify mechanisms of change and effects of mindfulness-based interventions for children and adolescents, especially since application of these types of interventions has rapidly increased across the past decade. This chapter synthesizes studies about the development of the original English version of the Child and Adolescent Mindfulness Measure (CAMM), as well as psychometric studies of authorized translations into Portuguese, Catalan, Dutch, Italian, and French. Good psychometric properties are shown for the original 10-item version of the CAMM, as well as for the other languages. Also some studies in samples with different cultural backgrounds have been carried out, as well as correlational and some intervention studies using the CAMM. Overall, the CAMM has been shown to be a short, user-friendly, reliable, and valid tool for mindfulness assessment in different stages of childhood, across different countries, languages, genders, socioeconomic, and cultural backgrounds.