If you are talking plants in general Bernadette, then I start cutting back about now. My hardy geraniums were cut back last week, my daisy types a few days ago and the group 3 clematis will be soon. As the others die off or go dormant they will follow. Some of course I don't cut back at all, just let them shrivel naturally then trim them. The trouble with many is that waiting until spring means that you cut off new growth. Not always a problem but really it depends on the species.

As Sarraceniac suggests, depends on the variety of plant. Herbaceous perennials will have died back down by November all on their own. There is an argument that you should leave the dead foliage in place and pull it off next spring because it gives some protection to the roots in the ground, and is a possible source of food for birds during winter, but that rather depends on how tidy you want your garden.
Any shrubs which flower in spring, or at least by mid to end of June, should not be cut now or you risk cutting off next year's flowers. Shrubs which flower later, or aren't grown for their flowers, can be cut in November, in fact, its probably better to do them then because they're almost dormant for winter and unlikely to respond by putting out new, soft growth which will be killed back when it gets cold.