The present invention is to provide a toner which has an excellent balance between heat-resistant storage stability and low-temperature fixability and which is excellent in printing durability even under a wide range of temperature and humidity environments from a low temperature and low humidity environment to a high temperature and high humidity environment. Disclosed is a toner for developing electrostatic images, containing colored resin particles containing a binder resin, a colorant, a softening agent and a retention aid, and an external additive, wherein the retention aid is a copolymer of at least one of acrylic acid ester and methacrylic acid ester and at least one of acrylic acid and methacrylic acid, and wherein the copolymer has an acid value of 0.5 to 7 mgKOH/g, a weight average molecular weight Mw of 6,000 to 50,000, and a glass transition temperature of 60 to 85° C.