A method of forming a core-shell polymer nanoparticle encapsulating an active agent is disclosed, including the use of a multi-solvent system in which to dissolve the active agent and a polymer prior to their precipitation using an antisolvent. The preferred use of an organic solvent system comprising two or more organic solvents allows for a high degree of control, as compared with the use of a single solvent, and enables the active agent to be precipitated more or less simultaneously with, or just prior to, the polymer.