The invention provides a method for preparing an aqueous dispersion of polymer particles comprising the following steps: (i) preparing a dispersion having a continuous aqueous phase, a dispersed organic phase comprising one or more ethylenically unsaturated monomers, and an amphiphilic RAFT agent as a stabilizer for said organic phase, and (ii) polymerizing said one or more ethylenically unsaturated monomers under the control of said amphiphilic RAFT agent to form said aqueous dispersion of polymer particles, novel amphiphilic RAFT agents for use in this method, novel RAFT agents useful in making these amphiphilic RAFT agents and methods for their manufacture.