The prevalence of childhood and adolescent obesity has increased and exacerbated during the coronavirus disease 2019 pandemic, both in Korea and globally. Childhood and adolescent obesity poses significant risks for premature morbidity and mortality. The development of serious comorbidities depends not only on the duration of obesity but also on the age of onset. Obesity in children and adolescents affects almost all organ systems, including the endocrine, cardiovascular, gastrointestinal, reproductive, nervous, and immune systems. Obesity in children and adolescents affects growth, cognitive function, and psychosocial interactions during development, in addition to aggravating known adult comorbidities such as type 2 diabetes mellitus, hypertension, dyslipidemia, nonalcoholic fatty liver disease, obstructive sleep apnea, and cancer. Childhood and adolescent obesity are highly associated with increased cardiometabolic risk factors and prevalence of metabolic syndrome. The risk of cardiovascular and metabolic diseases in later life can be considerably decreased by even a small weight loss before the onset of puberty. Childhood and adolescent obesity is a disease that requires treatment and is associated with many comorbidities and disease burdens. Therefore, early detection and therapeutic intervention are crucial.