Assieu is located some 30 km south by south-east of Givors and 8 km east by north-east of Saint-Maurice-l'Exil. Access to the commune is by the D131 road from the D134 in the south passing north then east in the commune to the town then north and continuing north-west to Route nationale 7. Apart from the town there is the village of La Charinaz in the east and Les Bruyeres in the west. The commune has large forests in the east as well as one in the west with significant residential areas and the rest of the land farmland.
The Vareze river forms the northern border and gathers many tributaries rising in the commune as it flows west to join the Rhône at Saint-Alban-du-Rhône.
In 2010 the commune had 1,316 inhabitants. The evolution of the number of inhabitants is known from the population censuses conducted in the commune since 1793. From the 21st century, a census of communes with fewer than 10,000 inhabitants is held every five years, unlike larger towns that have a sample survey every year.[Note 1]
^At the beginning of the 21st century, the methods of identification have been modified by Law No. 2002-276 of 27 February 2002, the so-called "law of local democracy" and in particular Title V "census operations" allows, after a transitional period running from 2004 to 2008, the annual publication of the legal population of the different French administrative districts. For communes with a population greater than 10,000 inhabitants, a sample survey is conducted annually, the entire territory of these communes is taken into account at the end of the period of five years. The first "legal population" after 1999 under this new law came into force in 1 January 2009 and was based on the census of 2006.