An organic light emitting display device including a pixel connected to a data line, a gate line group and a reference line, and a sensing unit, the pixel comprising: an organic light emitting device (OLED); a driving transistor (Tdr) configured to control a current flowing in the organic light emitting device (OLED) and including first and second gate electrodes (g1_Tdr, g2_Tdr) overlapped with each other, with a semiconductor layer provided therebetween; a first switching transistor configured to selectively supply a data voltage (Vdata) supplied to the data line to a first node connected to the first gate electrode (g1_Tdr); a second switching transistor configured to selectively supply a sensing voltage (Vsen) to the second gate electrode (g2_Tdr); a third switching transistor configured to selectively connect a second node connected to a source electrode (s_Tdr) of the driving transistor (Tdr) to the first node; a fourth switching transistor configured to selectively connect the reference line to the second node; a first capacitor connected between the second gate electrode (g2_Tdr) and the second node, the first capacitor is configured to store a threshold voltage (Vth) of the driving transistor (Tdr); and a second capacitor connected between the first and second nodes (n1, n2), the second capacitor is configured to store a difference voltage of the first and second nodes (n1, n2), wherein the display device is configured to drive the pixel during a reset period and a subsequent external sensing period in accordance with an external sensing driving mode such that, during the reset period, the first switching transistor is turned off, the second to the fourth switching transistors are turned on and a reference voltage (Vref) is supplied to the reference line so that voltages of the first and second nodes are reset to the reference voltage (Vref) and so that the first capacitor is reset by a difference voltage between the sensing voltage (Vsen) supplied to the second gate electrode and the reference voltage (Vref) supplied from the reference line to the second node, and, during the external sensing period, the first switching transistor is maintained in a off state, the second to the fourth transistors are maintained in a on state and the reference line is connected to the sensing unit, so as to drive the driving transistor in a source follower mode by the sensing voltage supplied to the second gate electrode and so as to configure the sensing unit to sense the threshold voltage (Vth) of the driving transistor (Tdr) through the reference line and to generate sensing data (Sdata) based on the sensed threshold voltage. an organic light emitting device (OLED); a driving transistor (Tdr) configured to control a current flowing in the organic light emitting device (OLED) and including first and second gate electrodes (g1_Tdr, g2_Tdr) overlapped with each other, with a semiconductor layer provided therebetween; a first switching transistor configured to selectively supply a data voltage (Vdata) supplied to the data line to a first node connected to the first gate electrode (g1_Tdr); a second switching transistor configured to selectively supply a sensing voltage (Vsen) to the second gate electrode (g2_Tdr); a third switching transistor configured to selectively connect a second node connected to a source electrode (s_Tdr) of the driving transistor (Tdr) to the first node; a fourth switching transistor configured to selectively connect the reference line to the second node; a first capacitor connected between the second gate electrode (g2_Tdr) and the second node, the first capacitor is configured to store a threshold voltage (Vth) of the driving transistor (Tdr); and a second capacitor connected between the first and second nodes (n1, n2), the second capacitor is configured to store a difference voltage of the first and second nodes (n1, n2), wherein the display device is configured to drive the pixel during a reset period and a subsequent external sensing period in accordance with an external sensing driving mode such that, during the reset period, the first switching transistor is turned off, the second to the fourth switching transistors are turned on and a reference voltage (Vref) is supplied to the reference line so that voltages of the first and second nodes are reset to the reference voltage (Vref) and so that the first capacitor is reset by a difference voltage between the sensing voltage (Vsen) supplied to the second gate electrode and the reference voltage (Vref) supplied from the reference line to the second node, and, during the external sensing period, the first switching transistor is maintained in a off state, the second to the fourth transistors are maintained in a on state and the reference line is connected to the sensing unit, so as to drive the driving transistor in a source follower mode by the sensing voltage supplied to the second gate electrode and so as to configure the sensing unit to sense the threshold voltage (Vth) of the driving transistor (Tdr) through the reference line and to generate sensing data (Sdata) based on the sensed threshold voltage.