According to some embodiments, a high bandwidth memory device includes a base die and a plurality of memory dies stacked on the base die and electrically connected to the base die through a plurality of through substrate vias. The base die includes a plurality of first input buffers configured to receive channel clock signals, channel command/addresses, and channel data from a plurality of first bumps connected to the outside of the base die, a plurality of second input buffers configured to receive test clock signals, test command/addresses, and test data from a plurality of second bumps connected to the outside of the base die, a monitoring unit, a plurality of first output buffers connected to the monitoring unit and configured to output monitored data from the monitoring unit to the plurality of second bumps, and a plurality of paths from the plurality of first input buffers to the monitoring unit. The plurality of second bumps are connected to receive test clock signals, test command/addresses, and test data from the outside of the base die during a first operation mode, and to receive monitored data from the plurality of first output buffers during a second operation mode.