The problem of traffic grooming is a crucial constituent in designing WDM networks and has been extensively studied in the literature. Although it is expected that a part of the future traffic of WDM networks will be multicast, most studies of traffic grooming have assumed only unicast traffic. The paper investigates the problem of grooming multicast traffic in WDM networks. More specifically, different sequential single-hop and multi-hop grooming approaches are studied and compared to traditional non-grooming approaches. We also propose a hybrid approach that utilizes the combined resources at the logical and optical layers.