The reptilian brain is considered to be the oldest part of the human brain. Reckoned to be hundreds of millions of years in development, it handles very basic things:

• That thing over there... Should I try to kill it, eat it, fuck it, ignore it, or run away from it?
• Voluntary tasks requiring sophisticated coordination, such as ambulation
• Regulatory tasks, such as the sensation of hunger, thirst, tiredness
• Metabolic control - being asleep or awake, restful or active; control of breathing, heart rate, and other involuntary systems such as the digestive tract

The following components perform functions including those associated with the reptilian brain:

• cerebellum
• medulla
• midbrain
• pons
• brain stem

Compare with the mammalian brain and neocortex, which presumably developed as our distant ancestors evolved from reptiles into mammals.

Portions of this are derived from http://www.brainchannels.com/evolution/reptiliantable.html