Detecting ENP/NM/MNs in water is a demanding task not only because of the extremely small size of the particles and their potential sequestration and agglomeration, but also because of their unique physical and chemical characteristics. The aim of this chapter is to critically review the state-of-art of ENP/NM/MN detection in the environment, highlighting the advances that have taken place in the last five years (up to 2018). The available analytical techniques used for the detection and characterization of nanoparticles in water including particle size analysis, particle fraction concentration counts, surface area analysis, morphology and particle chemical composition analysis will be outlined and critically analyzed to describe advantages and pitfalls. Sample preparation aspects, imaging techniques (electron microscopy, scanning electron microscopy or X-ray microscopy), separation methods (flow field fractionation, liquid chromatography, hydrodynamic chromatography) and detection/characterization techniques (e.g. single particle inductive coupled plasma, mass spectrometry) have been discussed in depth. This chapter also introduces the established workflows to increase the knowledge on the fate, behaviour, disposition and toxicity of these ENP/NM/MNs in the environment.