Are activist investors good or bad for the firms they target? They hunt down companies they think are underperforming. They buy a stake in the business, then lobby for change. Critics say activists want to make a fast buck and then head for the exit. But you could regard these investors as doing a valuable service - challenging poorly performing company boards and making more profit for shareholders. Top UK names like Rolls Royce and John Menzies have been affected. Explore the world of activist investors with Evan Davis. Joining him will be: activist investor, Harlan Zimmerman, senior partner at Cevian Capital; Chris Walton, a company chairman and non-executive director; and Sacha Sadan, director of corporate governance at Legal and General Investment Management.

Producer: Lesley McAlpine.