Children account for 30% of visits to emergency departments, and approximately 5% of these children have serious illness requiring immediate intervention. Over the past decades, as medical knowledge and application have eradicated many illness and rendered others curable, trauma has emerged as the leading cause of morbidity and mortality after the first year of life. However, all children remain vulnerable to infection and its consequences in the first year and beyond the first year. Additionally, over this time span, there are more children with chronic complicated health problems who survive yet remain dependent on sophisticated medical care. Intuitively, prompt recognition of the very ill child and the execution of the immediate and necessary interventions may be life saving. The ability to accomplish this requires a knowledge of the common culprits resulting in serious illness, an understanding of how they manifest in the physical examination, and an array of technical skills utilizing appropriate specially sized equipment. Furthermore, an understanding of child development and the ability to interpret the physical examination of the children of different ages is essential. This article will attempt to simplify this seemingly overwhelming task by considering a common thread in all critical illness.