In the battle for influence, public affairs professionals make crucial strategic decisions every single day. "Should we go public with this case?" "Who are we going to lobby, and how?" "Should we form a coalition with other organisations?" Public affairs professionals often make these decisions based on their experience or their gut feeling. In practice, lobbying is often more of an art than a science. It is an intuitive and creative process, rarely involving any insights rooted in science. And yet many public affairs professionals are faced with uncertainty about the added value of their activities. "Does what we do really matter?" "What kind of impact do we have?" "Are we making the right strategic decisions?" Some colleagues seek to compensate for these doubts with an overwhelming dose of self-confidence. An experienced lobbyist recently said during a lecture: "The day I can measure my influence is the day I can double my rates." Other public affairs professionals are a little more modest and try to assess their impact with key performance indicators. They systematically review the lobbying tactics used. This systematic approach has gained a lot of traction in recent years. The smoky back rooms, the cigars, and whisky of the past are now giving way to evidence-based lobbying, based on facts, building a bridge between art and science of lobbying.