Attention Deficit Hyperactivity Disorder (ADHD) is classified within the family of neurodevelopmental disorders (NDDs), sharing the category with conditions such as Autism Spectrum Disorder, Specific Learning Disorders, Developmental Coordination Disorder, and Gilles de la Tourette Syndrome, among others. NDDs often coexist with one another and with various psychiatric or non-psychiatric conditions, which can obscure the primary disorder's diagnosis, especially when considering the patient's age and gender. Psychiatric comorbidities encompass both internalizing disorders, such as anxiety and depression, and externalizing disorders, including oppositional defiant disorder and conduct disorder. While daydreaming and emotional dysregulation are frequently observed in children and adults with ADHD, they do not constitute diagnostic criteria for the disorder but rather comorbid features. The concomitant presence of these disorders can create complex clinical pictures, necessitating nuanced and careful management strategies. Comorbidities extending beyond psychiatric realms include various sleep disorders (disruptions in circadian rhythms, bruxism, enuresis, restless legs syndrome, among others), neurological conditions such as epilepsy and migraine, as well as atopic conditions, inflammatory and autoimmune diseases, and certain genetic syndromes, but not only. In children younger than six years old – recognizing that ADHD can be considered for diagnosis at this early stage – the manifestations of various disorders frequently intertwine, culminating in an overarching phenotype. As children age, these symptom clusters tend to become more distinct and individualized. In contrast to males, females diagnosed with ADHD are less likely to exhibit conspicuous signs of hyperactivity and impulsivity. They might adopt compensatory behaviors to obscure or diminish the manifestation of their symptoms. Furthermore, females have an increased likelihood of developing comorbid internalizing disorders, such as anxiety and depression, as opposed to externalizing disorders, which may contribute to the underdiagnosis of ADHD in this population. The prevalence of ADHD is non-negligible, affecting an estimated 2% of pre-schoolers, 4% of school-aged children, 3% of adults, and 1% of the population aged over 60, implicating more than two million individuals in France alone. Diagnostic criteria are exclusively clinical and have remained unchanged for over two decades in international classifications such as the Diagnostic and Statistical Manual of the American Psychiatric Association and the International Classification of Diseases by the World Health Organization. The absence of a reliable biomarker to confirm or refute the diagnosis underscores the challenges in ADHD identification. The ramifications of diagnostic delay or oversight are profound, potentially leading to dire consequences for affected children and their families, including elevated risks of suicidal behavior in adolescents and adults and increased potential for legal entanglements, even incarceration. This review article delineates the systematic approach to ADHD screening and diagnosis, including the identification of hallmark symptoms and their impacts, differentiation from other diagnoses, and the contemplation of ancillary assessments that may be warranted. These components are pivotal for informing therapeutic strategies, patient management, and the oversight of pharmacological interventions and their adverse effects. We furnish a compendium of evaluative scales and semi-structured interviews for clinical utility, enabling practitioners to elect tools best suited to their expertise and the diagnostic or evaluative objectives at hand. In sum, the diagnosis of ADHD is predicated on a methodical clinical assessment executed by a practitioner well versed in typical child development and neurodevelopmental disorders, eschewing reliance on supplemental biological, electrophysiological, neuropsychological testing, or neuroimaging.