In this work, plant extracts and compounds as a source of biofilm inhibiting substances were analyzed, with a focus on seagrasses and mangroves. To have access to fresh plant material, and to limit plant collection in the wild, mangrove cultivation in the greenhouse was studied. Good growth and successful propagation of Avicennia germinans and Laguncularia racemosa was achieved. Bruguiera cylindrica was growing very slowly and could not be propagated. The composition of secondary metabolites present in greenhouse grown A. germinans was comparable to plants collected outdoors in Guatemala. The internal transcribed spacer (ITS) as a genetic marker was shown to be a useful tool in the clear species identification of mangroves. Different microbial biofilm assays were carried out to study biofilm inhibitory actions of plant extracts. In a biofilm assay specific for Escherichia coli macrocolony growth and extracellular polymeric substance (EPS) production, different tea varieties and one flavonoid were screened in a first approach. Green tea and hawthorn tea as well as the flavonoid taxifolin showed good inhibitory activities. Three seagrass species, namely Enhalus acoroides, Halophila ovalis and Halodule pinifolia were tested in different biofilm assays on E. coli and Candida albicans. E. acoroides showed to be a promising source of biofilm inhibiting compounds, which are also able to induce cell dispersion from C. albicans biofilms.In preliminary experiments with mangrove extracts on biofilm inhibition, extracts of L. racemosa were most effective. Crude extracts of L. racemosa were able to reduce biofilm formation of E. coli, C. albicans and Candida glabrata in microtiter-based assays. To identify the active compounds, fractions of the crude extract enriched for phenolic compounds were tested. Here, two fractions inhibited C. albicans biofilm adhesion to 51 and 57%, respectively, compared to the positive control. The substances in the fractions were identified as ellagitannins and one gallotannin by liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR) spectroscopy. Obtained mass-spectrometric fragmentation patterns were compared to databases and the literature. According to this study, mangroves and seagrasses can be considered as a source of biofilm inhibiting compounds.