A main finding from this reserach is that regimes in most cases matter but environmental problems are seldom fully solved by the regimes in question. Turning to the empirical reality, hundreds of multilateral environmental agreements (MEAs) have been established since the late 1960s. Economic globalization, population growth, hyper-consumerism and the rise of the emerging economies like China and India are among the factors offsetting the positive effects of international environmental regimes. The study of international regimes has been conducted for more than two decades and they have become increasingly sophisticated over time. However, there are still deep-seated disagreement in the reserach community as to how precisely effectiveness can be measured. Over time empirical research on effectiveness has been reduced and replaced by issue areas like regime complexity and the role of partnerships. Here it is argued that however important these new fields may be they can not replace effcetivness studies. New empirical reserach on effcetivenss is therefore needed, not only updates of previous studies but novel approaches is also called for.