Healthy grapes have a complex microbial ecology including filamentous fungi, yeasts and bacteria. The variety and population of microorganisms present on the grape surface will depend on many factors. However, high sugar concentrations on the berry surface of damaged grapes will especially favour the increase of undesirable microorganisms, such as acetic acid bacteria and wine spoilage yeasts. These microbes are transferred to the must during processing and can have a profound influence on wine composition, flavour and quality. Populations can increase with extended duration of pre-fermentation operations, such as grape transportation, cold soaking, cold storage of juices or grape dehydration. Winemaking strategies to reduce the population of unwanted microbes usually include the use of sulphur dioxide. With the increasing demand for low sulphur dioxide wines, alternative strategies such as bioprotection are explored.