When the COST Action 919 started to investigate the role of melanoidins in food and health in 1999, the chemical structures of dietary melanoidins were poorly defined and hardly anything was known about structure-specific health effects of this chemical class. In addition, the degradation of melanoidins in the gut and their absorption and function or that of any of their degradation products had not yet been reported. In the past five years, results from in vitro studies demonstrated that at least some of the dietary melanoidins are degraded by intestinal microorganisms, possibly influencing their growth rate. The absorption and excretion rates of individual Maillard reaction compounds and melanoidin structures have been investigated in animal studies. These studies show that at least 30% of the ingested dose of low-molecular-weight compounds are absorbed. Structure-specific health-promoting effects of newly identified compounds have been described by means of their antioxidant and chemopreventive activity in cell culture investigations as well as in animal feeding studies and human trials. Harmful effects of dietary melanoidins have been investigated in the context of their ability to promote glycation reactions in vivo, which are involved in the progression of several diseases, such as diabetes mellitus, cardiovascular complications, and Alzheimer's disease. Toxicological studies were performed showing that melanoidin structures can not be classified as potent dietary mutagens or genotoxins. Thus, substantial knowledge on the health effects of melanoidins has been gained within COST Action 919. But still, further studies are needed to distinguish between chemically identified harmful and health-beneficial melanoidins.