Evolutionary Trends in Body Size -- Sexual Dimorphism in the Growth of Homo sapiens: Facts, Inferences and Speculation -- Genetic Control of Size at Birth -- The GH/IGF-1 Axis: Insights from Animal Models -- Downstream Mechanisms of Growth Hormone Action -- Growth Hormone Receptor Signaling and Differential Actions in Target Tissues Compared to IGF-I -- IGF-I and Brain Growth: Multifarious Effects on Developing Neural Cells and Mechanisms of Action -- IGF-I Deficiency: Lessons from Human Mutations -- Putting IGF-I Biology into a Clinical Perspective -- IGF Resistance: The Role of the Type 1 IGF Receptor -- The Importance of the National Cooperative Growth Study (NCGS) -- Why we are Treating Children with Growth Hormone: Lessons from the French Registry. Growth is a complex process that is essential to life. Not only does size play an important role in the process of cellular proliferation, but body size is also a critical factor in determining which organisms live longer. In mammals, the major factors involved in the regulation of body growth are known: insulin-like growth factors (IGF) are key regulators of somatic growth. Growth hormone (GH), secreted by the pituitary gland, directly regulates circulating levels of IGF-I, which is the major coordinator of spatio-temporal growth of the organism. In humans, growth involves a number of specific characteristics not found in other species. These include rapid intrauterine growth, deceleration just after birth, a mid-childhood growth spurt, a second deceleration before puberty, an adolescent growth spurt, and finally full statural growth, which is seen somewhat later. The combined knowledge concerning the endocrine and paracrine aspects of growth have led to the introduction of treatment regimens, most effective in GH-deficient children.