In the process of becoming independent from fossil hydrocarbon resources bio-based plastic is one option, being also favoured by programmes of the European Commission. To achieve maximum sustainability bio-based polymers that are also biodegradable are entering the market, with nationally differing regulation. It is wise to assess the risk of these new materials especially in fields where their use is intrinsically linked to a loss to the environment (e.g. by conversion to micro-plastics through wear), or where unintentional littering is probable. Thus, standard tests (e.g. ISO) are needed to validate the claim of a material being “biodegradable” for consumer safety and environmental impact. Here we give an overview of current marine standard tests and present results from the EU-funded project Open-Bio on the development of tests of biodegradability under marine conditions. We present feasible field test systems for three coastal scenarios: plastic being buried in intertidal beach sand (eulittoral test), plastic floating in the shallow water column (pelagic test) and plastic sunken to the sandy sublittoral (benthic test). The field tests were optimized to a state that the test material could be observed in situ over a time of up to 2 years without loss of larger fragments, and marine disintegration could be followed for common biodegradable polymers. Degradation rates for the tested polymers are given. A set of mesocosm experiments simulating the same three habitats supported the field research in a semi-controlled setting of environmental conditions, and allowed to critically evaluate field and lab test results, leading to an environmentally relevant test scheme. Our outlook shows the next steps of test development needed to provide a comprehensive toolset to cover the majority of conditions in which plastic is found in the marine realm.