  Exact solutions for nonexpanding impulsive waves in a background with nonzero
cosmological constant are constructed using a `cut and paste' method. These
solutions are presented using a unified approach which covers the cases of de
Sitter, anti-de Sitter and Minkowski backgrounds. The metrics are presented in
continuous and distributional forms, both of which are conformal to the
corresponding metrics for impulsive pp-waves, and for which the limit as
$\Lambda\to 0$ can be made explicitly.
