The process whose specific outcome is the progression of the subthalamus over time, from its formation to the mature structure. The subthalamus is the anterior part of the diencephalon that lies between the thalamus, hypothalamus, and tegmentum of the mesencephalon, including subthalamic nucleus, zona incerta, the fields of Forel, and the nucleus of ansa lenticularis. [ GOC:cls GOC:jid http://openisbn.com/search.php?q=0838580343&isbn=1/ GOC:dgh GOC:dph http://purl.obolibrary.org/obo/go/references/0000021 ]