The present invention provides methods and compositions for increasing seed size and/or weight, embryo size and/or weight, and cotyledon size and/or weight. The methods comprise expression of a cytokinin oxidase or expression of another protein that reduces the level of active cytokinins in plants or plant parts. Methods and compositions for increasing seed yield are also provided. The invention also relates to isolated plant cytokinin oxidase proteins, nucleic acid sequences encoding cytokinin oxidase proteins as well as to vectors, host cells, transgenic cells and plants comprising such sequences.