Plastic production has exponentially increased over the past years, exceeding 360 million tonnes annually. The high production rate coupled with high plastic durability and poor waste management leads to constant leakage and accumulation of plastic waste in aquatic ecosystems worldwide. In the marine environment, the fragmentation of larger plastic items leads to the formation of smaller fragments of micrometres (microplastics) or nanometres (nanoplastics) in length. Subsequently, micro- and nanoplastics are highly bioavailable to marine organisms and their effects have been extensively reported in the literature causing, among other things, physical damage, oxidative stress, disruptions in reproduction and mortality. Some of the contaminated marine species are intended for human consumption – seafood, which can potentially constitute a leading pathway of human exposure to plastic. Thus, the present project aimed to understand the extent of plastic contamination in marine organisms. In Chapter 2, a literature review was conducted to summarise existing information on the accumulation, fate and depuration of micro- and nanoplastics in different organisms, from marine invertebrates to humans, with a focus on aquatic species. It was found that, although most of the available data was on marine invertebrates, little was known on the accumulation, uptake and depuration kinetics of plastic particles in different tissues of high commercial value seafood species. Based on the knowledge gaps identified in Chapter 2, in Chapter 3 a quantitative method for plastic quantification in seafood was developed. Edible portions of seafood species with high economic value: oysters, prawns, squid, crabs and sardines were analysed. This was achieved through the development of a new method – Accelerated Solvent Extraction (ASE) coupled with Pyrolysis Gas Chromatography Mass Spectrometry (Py-GC/MS) - for identification and mass-based quantification of five selected plastics commonly found in the marine environment: polystyrene (PS), polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), and poly (methyl methacrylate) (PMMA). The results of this assessment revealed that the plastic content found across the different species analysed is highly variable, with sardines having the highest total plastic concentration, followed by crabs, oysters, prawns and squid. PVC was the only plastic present in all species analysed and PE the polymer found at the highest concentration across samples. Potential sources of plastic contamination to seafood are discussed. Following validation of this novel method, in Chapter 4, size fractionation of the plastic content in field-deployed oysters (> 22 µm; 1 - 22 µm; 150 µm in size. The Py-GC/MS analysis revealed that the 1 – 22 µm and the nano-sized (< 1 µm) fractions had the highest total plastic content, representing the size fractions commonly not reported in most studies assessing microplastics contamination in seafood. After assessing the nano-sized fraction in a species of high commercial value, in Chapter 5 we aimed to track nanoplastics inside live oysters (Crassostrea gigas) to understand their accumulation and depuration kinetics in different tissues. This was accomplished by using two types of polystyrene nanoparticles doped with palladium (Pd NPs) – Smooth and Raspberry - which enabled their unequivocal detection in the tissues by standard methods of trace metal analysis such as Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Transmission Electron Microscopy (TEM). After 6 days of exposure to the particles and 30 days of depuration, the Pd NPs were actively taken up by the gills in both treatments. Results reveal that the digestive gland was the target tissue of accumulation, with a faster elimination for the Smooth treatment and an almost complete depuration for the Raspberry treatment after 30 days. The results from this chapter provide useful information in terms of assessing a depuration time for shellfish concerning food safety. Overall, this thesis provides knowledge on the accumulation of plastics in marine organisms. For the first time, plastics detected in edible portions of high commercial value seafood are reported on a mass concentration basis. Size fractionation and quantification of oyster’s samples revealed the significant “missing” fraction of plastic - nanoplastic, which is not commonly quantified in most studies. The uptake and depuration kinetics experiments indicated that a depuration period for shellfish might be beneficial to avoid human exposure to plastic. Future research on improving methods of detection with a focus on nanoplastics and understanding and identifying possible sources of plastic contamination in seafood are imperative for advances in food safety regarding plastic contamination. Once techniques are established, collaboration between Research centres are recommended, so standard protocols for plastic measurement can be established.