Primary care providers and parents rely on developmental milestone checklists as tools for tracking a child’s development. However, there are multiple published milestone checklists that vary in their structure and content. Our team systematically evaluated the consistency and variability between four commonly used milestone checklists. We found that there was limited overlap in the content across these four checklists. In addition, the milestones shared across checklists were inconsistent in their estimated ages of when milestones should occur. Our findings highlight the need for objectively defined and accurately normed developmental milestones. Read the article.