This paper investigates the contribution of ICT to knowledge sharing in communities of practice. A theoretical model is built that identifies the possible influence of ICT on the extent to which knowledge is shared within a community, as well as a number of variables that determine the extent to which this contribution is realized. This theoretical model was tested within two ICT-facilitated communities for professionals in the area of working conditions. The results of these case studies show that ICT's most important contribution to knowledge sharing in communities consists of the realization of a shared information base (communality) and facilitating communication independent of time and place (connectivity). The results also show that trust among members of a community, and their identification with the community, are important influences on knowledge sharing. Task interdependence and the community's information culture are also identified as important influences.