Milk proteins and milk protein-derived peptides have been widely studied for their health enhancing properties. This chapter presents the updated scientific knowledge on the bioactive properties of milk protein-derived peptides. The different bioactive properties which have been attributed to milk protein-derived peptides are discussed. These include mineral binding, cardioprotective, antidiabetic, satiating, opioid, antimicrobial, immunomodulatory, anticancer and antioxidant activities. The structure-function relationship is presented for the aforementioned bioactive properties based on current scientific knowledge. For each bioactive property, the data obtained in vitro is discussed, followed by an analysis of the information obtained from animal and human intervention studies. To date, most studies have been conducted in vitro. However, an increasing number of in vivo studies testing the efficacy of milk protein-derived peptides are being conducted. In certain instances, the in vivo studies have confirmed the bioactivity of specific milk protein-derived peptides or milk protein hydrolysates. However, conflicting data still exist in the scientific literature, which demonstrates that the bioactive properties observed in vitro do not always translate in vivo. Detailed knowledge of the peptide sequences responsible for the bioactive properties, together with a better understanding of the bioavailability and stability of these peptides in vivo may help to enhance the development of milk protein hydrolysates with health promoting capabilities in humans. Ultimately, this may lead to the approval of health claims by the relevant regulatory agencies.