Additives, including nucleating agents, have been used to regulate the solidification process of (semi-)crystalline polymer solids and thus control both their crystallite dimensions and shape. Here, we demonstrate that minute amounts (0.1-1 wt%) of commercially available nucleating agents can be used to efficiently manipulate the solidification kinetics of a wide range of organic semiconductors--including poly(3-alkylthiophene)s, the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13-bis(triisopropyl-silylethynyl) (TIPS) pentacene--when processed from the melt, solution or solid state, without adversely affecting the semiconductors' electronic properties. Heterogeneous nucleation increases the temperature of and rate of crystallization of poly(3-alkylthiophene)s, permits patterning of crystallites at pre-defined locations in PCBM, and minimizes dewetting of films of TIPS-pentacene formed by inkjet printing. Nucleating agents thus make possible the fabrication of thin-film transistors with uniform electrical characteristics at high yield.