This article focuses on the interaction between parliamentary committees and external actors. How is the interaction organised, and how does it influence which interests are voiced? The authors show that institutional variation in procedures for calling witnesses and variation in committee agendas influence both the composition of actors and the concentration of evidence. By composition of actors, they refer to the set of different actor types involved. By evidence concentration, they refer to the extent to which evidence is provided by a relatively small share of active actors. The study is based on a new data set of all contacts between parliamentary committees and external actors in one year across three countries: the United Kingdom, Denmark and the Netherlands. Interestingly, the findings show that procedures of invitation rather than open calls increase the diversity of actor composition and decrease the concentration of actor evidence. This, however, comes at a cost, since the overall volume of contacts is reduced.