1.1. Nematodes in generalCurrently, nematodes are considered as one of the most numerous Metazoa on our planet. They can be either free-living, plant-parasitic nematodes or animal parasites. Different groups of nematodes have adapted to almost every habitat through the evolution over time. To date, the phylum Nematoda consists of about 27000 described species. However, the prediction of the real nematode number can up to a hundred million. Plant-parasitic nematodes are one of the most important pests due to their widespread distribution, host range and potential damage on various host plants. Currently, the number of described plant-parasitic nematodes is around 4100 species over the world (Perry & Moens, 2013).1.2. Nematofauna in BelgiumBelgium has a long “nematological tradition” with the relatively well-studied nematofauna. Recently, Steel et al. (2014) provided an updated list of Belgian nematofauna with 418 species, 127 of them are new compared to the lists of Coomans (1989) and Bert et al. (2003). According to Sewell (1970), Moens and Hendrickx (1990), Subbotin et al. (2000), Damme et al. (2013), Maria et al. (2015), Qing et al. (2015), Troccoli et al. (2016), Consoli et al. (2017), Janssen (2017), and Slos et al. (2017), the list of 1 free-living and 14 plant-parasitic nematode species should be added to Belgian nematofauna comparing with Steel et al. (2014).It is easy to recognize that the new species descriptions for Belgian nematofauna list are being added year by year. In addition, the previous data of plant-parasitic nematodes in Belgium were mainly from ‘classic’ biotopes with a strong dominance of agricultural and horticultural ecosystems (Bert et al., 2003; Steel et al., 2014). Therefore, in order to obtain a more comprehensive overview of the Belgian nematode diversity, it is necessary to investigate nematodes from various habitats including agricultural, horticultural, coastal line and forest ecosystems.1.3. Taxonomy of nematodes To assess biodiversity, to understand species distribution and to understand community structures and ecosystem functions, the taxonomy of nematodes is really important. In the past, nematode taxonomy was generally limited to ?-taxonomy that was mostly in view of morphology. However, the basic and conserved morphology of nematodes sometimes lead to misidentification. Moreover, homoplasy or convergent evolution, that occur frequently at all taxonomic levels, can be another difficulty in nematode identification. Hence, morphological characters alone are insufficient to resolve all the relationships, and this has resulted in controversial problems in nematode classification (Coomans, 2000).Currently, the shift from using phenotypic methods to using molecular approach is becoming more prevalent in nematology. Various molecular techniques such as protein-based analysis, quantitative polymerase chain reaction (qPCR), restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) analyses were used successfully for nematode identification. However, amplification and sequencing of rDNA and mtDNA genes have become the most reliable source for enhancing our comprehension of evolutionary and genetic relationships (Meldal et al., 2007).Similar to morphological approach, molecular one has its own limitations ranging from the quality of sequences to the methods to analyze the evolution and relationships. In order to avoid the misidentifications and the appearance of mislabeled sequences on GenBank as well as other limitations of the molecular approach, the combination of DNA sequences and morphological characteristics is desperately needed (Janssen et al., 2017).This study focuses on the investigation of plant-parasitic nematodes from all the neglected biotopes and this will provide a more detailed description of plant-parasitic nematode biodiversity in general and in Belgium particularly. The combination of molecular and morphological data in classification will contribute the knowledge to understand the controversial taxonomical problems as well as the phylogenetic relationships.