The massive accumulation of gelatinous material at and below the seawater surface occurred in Northern Adriatic, at a frequency and with such an extension to cause serious environmental and economic damages. The present review describes the updated knowledge and considers the main hypotheses on mucilage formation. They focus, in turn, on organisms that produce mucilage, on the particular hydrodynamic of the North Adriatic basin, on the role of nutrients, but the trigger mechanism has not yet been really understood. Finally, the theoretical framework recently proposed by Azam et al. is presented. In the organic matter continuum view, organisms of the microbial loop just represent a part of the whole picture from which specific mechanisms of interactions are examined and few important pathways are outlined for future research.