Plastic litter is entering and accumulating in our oceans and can be found in the marine environment all over the globe. When entering marine waters, plastics as any other surface, is rapidly colonized by a plethora of organisms, which form dense biofilms on the plastic surface, the so-called “Plastisphere”. Despite growing concerns about the ecological impact of plastics on the marine environment during the last decade, the number of studies addressing Plastisphere-related questions remains limited. This thesis aimed to tackle this knowledge gap by comprehensively describing and analysing specificities of Plastisphere communities attached to chemically distinct plastic types. The specificity of mature Plastisphere communities was investigated on nine chemically different plastic types, and compared to the inert control substrate glass. The Plastisphere communities attached to diverse plastic types were found to be distinct from glass-associated communities. A more general marine biofilm core community serves as shared core among all tested plastic types and glass, rather than specific Plastisphere communities. The general heterogeneity of eukaryotic communities was much higher, indicating that the term Plastisphere is valid for mature prokaryotic biofilm communities, but may not be for eukaryotic ones. This work also showed that the prokaryotic shared core of the various mature Plastisphere communities are rather substrate unspecific, pointing towards the importance of rather rare species in plastic associated marine biofilms. A high-pressure water Jet treatment technique was developed to remove the cohesive layer of mature biofilms, while the adhesive layer remains on the plastics surface . It was shown that tightly attached microorganisms might account rather to the rare biosphere in mature Plastisphere communities, which suggests the presence of plastic “specific” microorganisms/assemblages. Due to their longevity, plastics could be transported over long distances in marine environments, and therefore may function as a vector for the dispersal of pathogenic species. To test this, plastic particles were collected in the North Sea and the Baltic Sea and screened for the presence of pathogens. Potentially pathogenic Vibrio parahaemolyticus were discovered on a number of microplastic particles, e.g. polyethylene, polypropylene and polystyrene. Mostly, this species co-occurred also in surrounding seawater, suggesting that seawater serves as a possible source for Vibrio colonization on microplastics. The confirmed occurrence of potentially pathogenic bacteria on marine microplastics highlights the urgent need for detailed biogeographical analyses of marine microplastics. The results from this thesis substantially increase our understanding of the diversity and specificity of Plastisphere communities. This thesis comprises a detailed and descriptive approach, which provides a fundamental knowledge basis for future research on Plastisphere questions related to e.g. potential biodegradation of marine plastics and the vector function for alien and potentially pathogenic species.