Many important economic and social situations are characterized by free rider incentives. Despite this, considerable cooperation is observed in reality. Many people vote, they join parties, they participate in collective actions, they contribute to public goods, give to charities and so on. This paper experimentally studies the role of "selective incentives" (Mancur Olson (1965)) in sustaining such voluntary cooperation. In particular, we are interested in the effectiveness of costly rewards and punishments to constrain free riding behavior. In our experiments subjects at a first stage had to make a contribution to a public good. Afterwards, at a second stage, all group members were informed about each others' decision. Dependent on the treatment, each group member then could either (i) punish each other group member, or (ii) reward each other group member, or (iii) both. Both punishment and reward were costly and never in a subject's material interest. Hence, rewards and punishments should be ineffective in influencing voluntary cooperation. In contrast to this prediction, we find that both punishments and rewards are highly effective in sustaining voluntary cooperation. However, cooperation is much more stable with the threat of punishment than with rewards. We also find that most punishment is for deviations below the group average contribution. With rewards, however, we find that predominantly those are rewarded who contribute about the same as oneself to the public good.