In angiosperms, a double-fertilization event leads to the formation of a diploid embryo and a triploid endosperm. In Arabidopsis and many dicots, seed development undergoes an initial phase of active endosperm proliferation followed by a second phase in which embryo grows to full size and replaces most of the endosperm volume at its maturity. Since the seed coat and endosperm growth in Arabidopsis precedes embryo growth, the major volume of the mature seed is largely attained before the enlargement of the embryo. Therefore, the seed size is coordinately regulated by the growth of the triploid endosperm, the diploid maternal ovule, and the diploid embryo. Recent studies have identified many new pathway components and revealed possible mechanisms that underlie seed development and size regulation in Arabidopsis. In this review, we shall discuss the regulation of endosperm proliferation by a few newly identified pathways involving transcriptional, epigenetic, and imprinting regulators, the regulation of integument or seed coat development by a few transcription factors, and the regulation of embryo proliferation by AP2-like and bHLH proteins and phytohormones.