Grapes are vigorous growers that generally need some pruning every year. Winter, before they sprout new growth, is the best time to prune them. You can cut back grapes severely if need be. There are several options when it comes to pruning styles. Do you want to grow the grapes to maximize fruit production, or would you rather simply keep them in bounds. Best fruiting comes from 2-year-old wood. Are the grapes growing on a trellis or sprawling over a pergola? How vigorous are the varieties of grapes that you have. All these are questions that need to be answered for me to provide detailed pruning instructions. Check reference books or contact your local cooperative extension office for publications on grape pruning techniques.