The main information point for the Carnac alignments is the Maison des Mégalithes, 1km north of Carnac-Ville, which explores the history of the megalithic sites and has a rooftop viewpoint overlooking the alignements. Sign up for a one-hour guided visit; times vary considerably depending on the time of year but they run several times a day (in French) during the summer. English tours are available once a week in July and August – call to confirm times.
The sites are fenced off with green metal fences so that the vegetation has a chance to regenerate, so certain areas can only be accessed by guided tour. From October to March you can though walk through certain parts (there are gaps in the fences you can walk through) – the Maison des Mégalithes has maps of what's open. The Maison des Mégalithes also runs Prehistoric Life Workshops (€11.50).