Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study interfaces. Recently, multiplex heterodyne-detected VSFG (HD-VSFG) has been developed, which enables the direct measurement of complex second-order nonlinear susceptibility [χ⁽²⁾]. HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of χ⁽²⁾ [Imχ⁽²⁾] obtained with this interferometric technique is the direct counterpart to the infrared [Imχ⁽¹⁾] and Raman [Imχ⁽³⁾] spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG [|χ⁽²⁾|²] spectra. The Imχ⁽²⁾ signal is obtained with a sign that contains unambiguous information about the up/down orientation of interfacial molecules. Furthermore, HD-VSFG can be straightforwardly extended to time-resolved measurements when combined with photoexcitation. In this review, we describe the present status of experiments and applications of multiplex HD-VSFG spectroscopy, in particular with regard to the orientation and structure of interfacial water at charged, neutral, and biorelevant water interfaces.