A curable composition to be cured to provide a cured product excellent in heat resistance and dielectric properties, a cured product of the curable composition, a printed wiring board, a semiconductor sealing material, and a build-up film using the curable composition. There is provided a curable composition containing an aromatic ester resin (A) and a maleimide compound (B), the aromatic ester resin (A) being an active ester resin that is a reaction product of a first aromatic compound having two or more phenolic hydroxy groups, a second aromatic compound having a phenolic hydroxy group, and a third aromatic compound having two or more carboxy groups and/or an acid halide thereof or an esterified compound thereof, in which at least one of the first aromatic compound, the second aromatic compound, and the third aromatic compound and/or the acid halide thereof or the esterified compound thereof has a polymerizable unsaturated bond-containing substituent.