This invention relates to monodisperse magnetic hydrogel polymer particles comprising a magnetic material and a polymer formed from (a) a hydrophilic vinylic monomer having a log Poct/wat (log P) of less than about 0.5; and (b) a crosslinker comprising at least two vinyl groups. The invention also relates to monodisperse coated hydrogel polymer particles comprising a polymer formed from (a) a hydrophilic vinylic monomer having a log Poct/wat (log P) of less than about 0.5; and (b) a crosslinker comprising at least two vinyl groups; and a coating. Also provided are methods of forming the monodisperse magnetic hydrogel polymer particles and monodisperse coated polymer particles.