The use of nanoparticles to make nanocomposite membranes for water treatment spans a range of development with the majority of materials existing at lab scale, some having been piloted, and a select few at commercial scale. As such, the use of nanomaterials in this field is still in the early stages. There is a large parameter space to explore, which includes the many types of polymers used in membranes, the wide range of membrane types used in water separations, the great variety of nanomaterials and their properties, and the multitudinous ways in which nanomaterials can be added to polymers. Determining which polymer nanoparticle combination will deliver substantially improvement over the bare polymer is challenging, but attempting to predict how nanomaterials will impact the desired membrane and flux characteristics when incorporated into a polymer and formed into a membrane is even more difficult due to the complex thermodynamics and kinetics that occur between polymers and nanoparticles during membrane synthesis. Further, optimizing the interactions of nanoparticles with polymer systems is not trivial, but is critical to translating the nanomaterial properties to the polymer matrices in order to produce enhanced membranes. These challenges are opportunities to produce nanocomposite membranes with enhanced practical applications.