As wind penetration levels on power systems increase worldwide, the dynamic characteristics of these systems are changing due to the displacement of synchronous generation. One issue, of particular concern, is the resulting reduction in system inertia. Modern, variable speed wind turbines are controlled by power electronics and so do not inherently contribute to the inertial response of the system. Such devices can however be fitted with a control loop which provides an active power response to significant frequency deviations, similar to the inertial response of fixed speed wind turbines and synchronous generation. However, the response of variable speed turbines is dependent on local wind speeds and so cannot be quantified deterministically by system operators. This paper examines the potential for wind generation to contribute to system inertial response and considers the aggregated inertial response capabilities of fixed speed and variable speed wind generation.