A method and apparatus for motion estimation in a sequence of video images is provided. Each field or frame in a sequence of video images is sub-divided into a plurality of blocks. Each block in each video field or frame has assigned to it a set of candidate motion vectors. The vector which produces a best match to a block in a previous field or frame, from the set of candidate motion vectors, is assigned to that block thus forming the motion vector field for the current video field or frame using the selected vector. The set of candidate motion vectors assigned to a block include one or more candidate vectors derived from an external source vector field and each such vector from an external source vector field is assigned a weighting that biases towards or away from the selection of that vector.