  Perturbation techniques can be used as an alternative to supercomputer
calculations in calculating gravitational radiation emitted by colliding black
holes, provided the process starts with the black holes close to each other. We
give a summary of the method and of the results obtained for various initial
configurations, both axisymmetric and without symmetry: Initially static,
boosted towards each other, counter-rotating, or boosted at an angle
(pseudo-inspiral). Where applicable, we compare the perturbation results with
supercomputer calculations.
