The farewell tour is one of the oldest tricks in the rock-star playbook. If fans are on the fence about whether or not to see a show, there's no better way to cajole them than by saying this will be their final chance. And even though everyone from Kiss to Cher, Ozzy Osbourne, Tina Turner and Phil Collins have continued to tour after their supposed farewell treks – sometimes just about a year later – the gambit continues to work. It's become an especially potent tool in recent years, as many key baby-boomer acts have reached their seventies and their threat to never tour again suddenly feels credible. Here's a guide to six of the biggest farewell tours hitting the road this summer.