Methods of Printing and Electrostatic Ink Compositions Here is described a method of printing on a plastic substrate, the method comprising: providing an electrostatic ink composition comprising a carrier liquid, and particles comprising a resin, and a slip agent dispersed in the carrier liquid; forming a latent electrostatic image on a surface; contacting the surface with the electrostatic ink composition, such that at least some of the particles and the slip agent are transferred to the surface to form a developed toner image on the surface; and transferring the toner image to the plastic substrate. Electrostatic ink compositions and plastic substrates are also disclosed.