Decades of research have determined that an animal`s brain structure and behavior are molded by experience. Expected experience that plays a critical role in early organization of the brain may be encoded via a process of overproduction of synaptic connections followed by the loss of those that are underutilized during a critical period. However, novel information may be encoded throughout life by the formation of new synapses as the individual animal is exposed to new environmental stimuli. Many laboratory species reared in complex environments or trained to perform complex tasks, regardless of the age when the altered experience is introduced, will exhibit an increase in the number of synapses per neuron as well as other anatomical differences from those reared in standard laboratory housing. Nevertheless, even though increased environmental stimulation may result in more "normal" anatomical and physiological development for that species, there is no conclusive evidence that enriched caging is essential or even that it increases well-being in laboratory rodents.