Cumulus cells directly surround oocyte to form cumulus oocyte complex (COC). Oocyte secreted factors act on cumulus cells to regulate the specific functions. Since oocytes have less glycolytic activity, the energy sources, such as piruvate or amino acid, are transferred from cumulus cells to oocyte via gap junctional communications, which are required for oocyte growth. After the LH surge, granulosa cells produce EGF-like factors that act on cumulus cells to induce cumulus expansion and oocyte maturation. EGF receptor expressed on cumulus cells up-regulates the ERK1/2-dependent pathway. The signaling pathway plays important roles in oocyte meiotic maturation and cumulus expansion (accumulation of hyaluronan-rich matrix within cumulus cell layers). During the fertilization process, the hyaluronan-rich matrix is broken down and the small hyaluronan fragments stimulate cumulus cells via the toll-like receptor (TLR) pathway to secrete the chemokine family. The secret chemokines act on sperm to induce sperm capacitation. The TLRs expressed on cumulus cells of ovulated COCs are also involved in innate immune functions that recognize bacteria in order to protect the oocyte from infection. Since cumulus cells regulate oocyte growth, oocyte maturation and the fertilization process, a study clarifying the role of cumulus cells might contribute to the technical development of in vitro growth or in vitro maturation of oocytes, and the regeneration of oocytes from stem cells.