The role of a facilitator is key to the effective establishment and maintenance of communities of practice (COPs). This paper presents challenges faced by facilitators in COPs based on a taxonomy of facilitation tasks inspired by facilitation research in the area of Group Support Systems (GSS). An online survey of the experiences of COP facilitators was conducted to discover which tasks were the most difficult and the most important for COP facilitation. The results show that tasks related to participation are considered by the majority of facilitators to be difficult as well as important. This and other findings can help facilitators in preparing for their role in COPs.