Electricity supply systems were not designed with wind power in mind and until recently wind turbines were not expected to provide much in the way of support services to grid networks. But with increasing proportions of wind power on transmission systems, times are changing. In nearly all of wind's major markets, grid codes are being adapted to take wind power into account. While wind is being exempted from some rules, tough new demands are also being made of wind turbine technology. We examine the market impact of the changes, starting with a walk-through of the basics.