###### Abstract  
A series of negative, attendant circumstances threaten the ecological equilibrium, the quality, and even the existence of the different aquatic ecosystems nowadays more than ever. In both fresh and marine waters, it is possible to find traces or significant amounts of contaminants produced by and for human activities, unpredictably affecting the different trophic levels. Together with the excessive drawings on freshwater reserves, driven by the rise in population and the drought crisis linked to the climate changes, the pollution of the tiny fraction of freshwater available to us urgently requires effective, sustainable, and innovative monitoring and remediation tools. In light of the above, this special issue was proposed and so far, 16 papers have been published in this Special Issue, fairly equally distributed between monitoring activities and remediation proposals. The first paper reported the application of a polar organic chemical integrative sampler (POCIS) to the monitoring, by passive sampling, of marine water employed in aquarium tanks [1]. The passive accumulation devices provide in situ integrative sampling, pre-concentration and purification steps, allowing the determination of time-weighted average (TWA) concentrations of trace levels in the supply water of hydrocarbons, DPS, 2,4-DTBP, BPA and the UV-filters OC and EHMC. The result highlighted the usefulness of pre-concentration in the passive sampling approach, which permitted enhanced sensitivity. In the paper [2], the performance of conventional monitoring of PCBs by water sampling followed by GC-MS/MS determination was compared with biomonitoring using the Baikal omul, _Coreguns Migratorins_. Comparative statistical analyses were carried out to study both the influence of the year, season, locations, layers of the water column on water samples values and that of the age and sex of omul on the ratio and concentrations of indicator congeners in fish tissues. The determination in water was simple, obtained by measuring only two indicator congeners. The PCB accumulation in the muscle tissue depended on their hydrophobicity and on the age of individuals. Hence, the ratio of congeners in fish tissues did not reflect their ratio in the water samples. By using a manipulative feeding experiment, possible damages to the intestinal microbial community and morphology of _Microterius salmonides_ produced by virgin polypropylene microplastics (MPs) were investigated [3]. The tested MPs were fibres, fragments, and films added into the water or to the feed. After a four-week trial, no significant differences were detected in weight gain (WG%) and specific growth rate (SGR). MP exposure had no statistically significant effect on the composition or diversity of the intestinal microbial community, although it could partly influence the intestinal morphology and the recombination process of the intestinal microbial community. In [4], the authors carried out a numerical study, reproducing the changes in 1,4-dioxane concentrations in groundwater observed in the monitoring wells of a waste dumping site. The quasi-3D flow and transport simulation model was useful in predicting the changes in pollutant amounts in the hydrologic environment, elucidating the transport under both natural and artificially induced water flows. The simulation revealed that the pumping actions could promote pollutant transport to areas where it would otherwise be difficult to reach. This phenomenon was unanticipated, and it would influence the groundwater treatment efficiency. In light of the differences that exist among pollutant mechanisms and among organism sensitivity, a careful evaluation of the suitability of three freshwater and marine microalgae and of _Vibrio fisheri_ bacteria in assessing the long-term toxicity of a set of benzo-fused nitrogen heterocycles was reported in [5]. No species-specific sensitivity for any whole family of pollutants was observed and the freshwater _R. subcapitata_ was the only alga affected by most of the compounds at the lowest concentrations.