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The wavelength-dependent mass absorption coefficient (opacity kappaabs) is a critical parameter in the determination of the total dust mass of IR-emitting dusty regions. The dust opacity shows marked variation with local conditions. Due to the incomplete understanding of the size, shape, composition and structure of dust grains, our knowledge of the long wavelength dust opacity is subject to large uncertainties. We apply the Kramers-Kronig relation to place a lower limit on the exponent index beta and an upper limit on the absolute value of the opacity (Section 2), if the dust opacity is described as a power-law function of wavelength (kappaabs ~ lambda-beta). Our current knowledge of the wavelength dependence exponent index beta (Section 3) and the absolute values of the opacity (Section 4) of interstellar dust is summarized in the context of interstellar grain models, laboratory measurements, and direct comparison of the short-wavelength extinction with the long wavelength thermal emission.

ACKNOWLEDGMENTS. I am grateful to C.C. Popescu and R.J. Tuffs for inviting me to give an invited talk at this stimulating conference from which I learned much. I also thank E. Dwek, C.C. Popescu, R.J. Tuffs, A.N. Witt, and E.M. Xilouris for helpful discussions.