A chip stack may include a first chip and a second chip stacked on the first chip. Each of the first and second chips may include a substrate having an active surface and an inactive surface opposite to the active surface; an internal circuit in the active surface; an I/O chip pad on the active surface and connected to the internal circuit through an I/O buffer; and a I/O connection pad connected to the I/O chip pad through the I/O buffer by a circuit wiring. A redistributed I/O chip pad layer may be on the active surface of the first chip, the redistributed I/O chip pad layer redistributing the I/O chip pad. The I/O connection pads of the first chip and the second chip may be electrically connected to each other by an electrical connecting part.