The foetal brain develops within few weeks from a thin cell layer to a gigantic and complex network with billions of neurons and trillions of connections. This process is influenced by environmental factors (e.g. maternal stress) from the beginning. Understanding of the developmental processes is the basis of prenatal medicine and psychology. Five steps can be related to fairly defined time periods: (1) From 7 to 22 weeks of gestation 20 billion neurons are produced and migrate to their final locations in the brain. (2) From 20 to 35 weeks the transient subplate structure lays the foundation of the cortex. (3) The organization of the neural network (nerve fibre and synapse formation) starts at 24 weeks gestation and continues throughout life. (4) Individual adjustment of the neural network by elimination of more than 50% of the neurons and circuits also starts at 24 weeks, and shapes the brain in three waves. (5) Myelination of axons begins during the last weeks of gestation and continues for decades.