The availability of powerful and affordable 3D PC graphics boards has made rendering of rich immersive environments possible at interactive speeds. The scene update rate and the appropnate behaviour of objects withm the world are central to this immersive feeling. This thesis is concerned with the behaviour computations involved in the flocking algorithm, which has been used extensively to emulate the flocking behaviour of creatures found in nature. The mam contribution of this thesis is a new method for hierarchically combining portions of the flocks into groups to reduce the cost of the behavioural computation, allowing far larger flocks to be updated in real-time in the world