This invention relates to monodisperse cross-linked hydrogel polymer particles comprising a polymer formed from (a) a hydrophilic vinylic monomer; and (b) a crosslinker comprising at least two vinyl groups. The invention also relates to monodisperse seed particles with a Z-average diameter of from 100 nm to 1500 nm that comprise a plurality of non-crosslinked oligomers of poly N,N-dimethylacrylamide. Also provided are methods of forming the monodisperse cross-linked hydrogel polymer particles and monodisperse seed particles.