This chapter illustrates water and wastewater treatment (WWT) technologies that support membrane separation. It covers a range of membrane filtration methods, from microfiltration to reverse osmosis, as well as membrane distillation and membrane-integrated hybrid process technologies. Following discussions on membrane materials, flow regimes, membrane modules, and transport mechanisms, various membrane-based processes are demonstrated through examples. Nanofiltration (NF) membranes are a critical technology in wastewater treatment. As a relatively recent advancement in membrane technology, NF membranes can be used with both aqueous and non-aqueous solutions. Their characteristics lie between Ultrafiltration (UF) and Reverse Osmosis (RO), utilizing both pore-size flow (convective) and solution-diffusion mechanisms. Membrane surface charges, often negative in NF membranes, play a crucial role in their function. NF technology is applied in various wastewater treatment processes across various industries, primarily for the selective removal of ions and organic substances. It is also employed in specific seawater desalination applications. This chapter highlights the key applications of NF membranes in water reuse, tertiary wastewater treatment, water softening, and desalination.