: Variations in weight, height, and head circumference that still fall within healthy percentiles.

The book, often searched for in format by medical students and professionals, focuses on the high degree of variation within "normal" development. Illingworth argued that practitioners must understand the vast spectrum of normal behavior to avoid over-diagnosing disorders in healthy children. Key themes covered in the text include:

: The "normal" range for walking, talking, and social interaction, highlighting that children develop at different rates.