Micro-oxygenation (MOX) is the process of introducing a low amount of oxygen during initial aging of red wine in the attempt to control desirable changes in its sensory properties. Although MOX can enhance the structure and colour of wine due to the reaction of phenolic compounds to form more stable polymeric structures, the reduction of astringency is less certain. It is also claimed that MOX might reduce excessively herbaceous character and reductive aroma of wine. MOX was developed during the early 1990s and is usually applied to red wines stored in steel tanks as alternative to oak ageing. The oxygen level into the wine over the course of treatment is the critical point to control the oxidation reactions that take place. Thus, the success of the MOX relies on the ability to control the rate of oxygen dosage. The MOX process is not well understood yet and represents one of the major controversial application of modern winemaking. The importance of oxygen level on the chemical and sensory changes of red wines is the central topic of this chapter.