The model year of a product is a number used to describe approximately when a product was produced. The model year and the actual year of production don't always coincide, however. For instance, in the United States, automobile model years traditionally start in the third quarter of the preceding year. In other cases products of a previous model year can continue production, especially if a newer model hasn't yet been released.

In the United States, government authorities allow cars of a given model year to be sold starting on January 1 of the previous calendar year.

The practice of identifying revisions of automobiles by their "model year" is strongest in the United States. Typically, complete vehicle redesigns of longstanding models occur in cycles of at least five years, with one or two "facelifts" during the model cycle, and are introduced at various times throughout the year. Additionally, introductions of new models are often phased in around the world, meaning that a "2004 model" of a particular vehicle may actually refer to two entirely different vehicles in different countries. Therefore, the more common practice for enthusiasts and motoring writers in other countries is to identify major revisions using the manufacturer's identifier for each revision - for instance, Holden Commodores, an Australian family sedan, are grouped into the following series: VB (introduced 1978) VC (1979), VH (1981), VK(1984), VL(1986), VN(1988), VP(1991), VR(1993), VS(1995), VT(1997), VX(2000), and VY(2002).