From July 1 2008, it is a legal requirement for all vehicles to carry one warning triangle and one reflective (fluorescent) safety jacket.
The safety jacket must be stored inside the car somewhere easily accessible such as under your seat or in your glove box but not in the boot. In the event of breakdown, emergency or stopping for any other situation, the safety jacket must be worn by the driver before getting out of the vehicle. The jacket must conform to EU Regulations, Directive 89/686/EEC, if the jacket bears the CE mark it should comply to this standard.
The warning triangle may be carried in the boot. The driver must immediately place the triangle at least 30 metres from the vehicle so that it can be seen by all approaching traffic. The triangle must conform to the Geneva regulation number 27, bearing a mark with an E and 27R.
Anyone found to not carrying a safety jacket and warning triangle could be subject to a fine between €90 and €135 from 1 October 2008.
Further information from the Sécurité Routière, in French, here
These items are also legally required in all vehicles in Italy and Spain and possibly other parts of Europe as well. You'd be advised to check first before driving through these countries.