This chapter describes the external and internal physical changes and the brain. The hormonal changes of puberty initiate drastic growth in the body and organs of adolescents. Recent advances in brain-imaging technologies such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) scans have contributed greatly to understanding of brain development in adolescence. Similar to what happens during infancy, the early adolescent’s brain begins a process of overproduction, which is an increase in neural connections in the brain’s gray matter. The development of gray matter follows a pattern of maturation from the back of the brain to the front of the brain. By eliminating unused synapses the adolescent brain becomes more efficient and is able to process mental functioning at an accelerated speed. The amygdala is a small, almond-shaped structure in the midbrain charged with emotional expression.