Sluggish Cognitive Tempo (SCT), also recently referred to as Cognitive Disengagement Syndrome (CDS), has gained increasing attention as a clinical construct distinct from but closely related to Attention-Deficit/Hyperactivity Disorder (ADHD). Originally described through symptoms of daydreaming, hypoactivity, mental fogginess, and lethargy, SCT/CDS is now recognized as a potential independent syndrome with unique developmental, cognitive, and psychosocial correlates. This chapter synthesizes historical, diagnostic, and empirical perspectives on SCT/CDS, tracing its evolution from a variant of ADHD to a candidate disorder with growing research support. Epidemiological data indicate that SCT/CDS occurs in approximately 2–8% of children and up to 7% of adults, with comorbidity rates with ADHD approaching 60%. While ADHD is strongly associated with externalizing behaviors such as hyperactivity and impulsivity, SCT/CDS is more consistently linked to internalizing symptoms, including depression, anxiety, social withdrawal, and emotion dysregulation. Neuropsychological findings reveal that SCT/CDS is not marked by broad executive dysfunction, but rather by selective deficits in sustained attention, vigilance, and metacognitive organization. Importantly, SCT/CDS appears to predict poorer academic performance, occupational difficulties, and lower quality of life, even after controlling for ADHD symptoms, underscoring its independent clinical relevance. This chapter also reviews current measurement tools, including validated child, adolescent, and adult scales, which demonstrate robust psychometric properties across cultures and informants. Preliminary treatment studies suggest potential benefits of stimulants, atomoxetine, and lisdexamfetamine, alongside psychosocial interventions such as cognitive-behavioral therapy, behavioral activation, and mindfulness-based strategies. However, no intervention has been developed specifically for SCT/CDS, highlighting the urgent need for clinical trials tailored to its symptom profile. In conclusion, SCT/CDS represents a clinically significant, functionally impairing syndrome that extends beyond ADHD. Clarifying its nosological status, refining assessment methods, and developing evidence-based treatment protocols constitute essential directions for future research and clinical practice.