Developmental disorders are diverse, common, and impairing; still, many clinicians lack comprehensive training in their assessment. This review presents thorough guidelines for the evaluation and diagnosis of common communication, sleeping, feeding, and elimination disorders that often onset in the early developmental period and that are commonly encountered in clinical practice. Thorough guidance on the evidence-based assessment of developmental disorders is critical, as they are prevalent, impairing, and commonly comorbid with other psychiatric disorders of childhood. This review is the first of its kind—providing critically needed, step-by-step guidance on the existing evidence-based methodologies and assessment tools available for diagnosis. This review also makes clear the dire need for further development and validation of relevant screening and diagnostic measure and calls for specific attention to the development of specific screening and diagnostic assessment measures for feeding disorders and elimination disorders in particular. Clinicians and researchers alike may find this article useful in guiding diagnostic, treatment, and research procedures.