To compete with high quality wines from abroad, the wines from Germany and Luxembourg need to develop a more characteristic and superior sensory profile. During spontaneous fermentation, which is increasingly applied by wine makers of the upper quality segment, the activity of the wine yeast Saccharomyces cerevisiae can slow down (sluggish fermentation) or even decline to zero (stuck fermentation). Some reasons for this unsuccessful fermentation, such as lack of nutrients, unfavourable temperature or pH as well as a low glucose/fructose ratio are known, but the problem can often not be solved by the application of classical measures such as temperature adjustment, the addition of nutrients or restarting fermentation with sparkling wine yeasts. In addition, wine makers of the upper quality segment avoid these procedures, since they could change the characteristic sensory profiles of the individual wines.We would like to investigate the so far overlooked problems during spontaneous fermentation which may be caused by interactions between the diverse species of the microbiota among themselves and influenced by certain chemical compounds produced during the fermentation process. Therefore, we will focus on the determination of the microbial diversity and succession at different stages of the fermentation process, their inhibitory and stimulating cell interactions, interactions of cells with selected must constituents, the impact of their physiological status (proteomics) and the chemical composition of the must during the complete fermentation period which can last from several weeks to several months. These investigations should light up so far not understood causes of sluggish and stuck fermentations during spontaneous fermentations applied for the production of high quality wines.