  We present and discuss the results of new multiple-scattering radiative
transfer calculations for three representative types of galactic environments,
filled with either homogeneous or two-phase clumpy dust distributions.
Extinction and scattering properties for two types of interstellar dust,
similar to those found in the average diffuse medium of the Milky Way Galaxy
(MW) and the Bar of the Small Magellanic Cloud (SMC), are considered. The
wavelength coverage extends from 1000 A to 30,000 A, with particular emphasis
on the rest-frame UV. This makes these models especially applicable to
starburst galaxies and Lyman-break galaxy samples. The examination of the
models concentrates on the study of UV/visual/near-IR reddening effects, the
wavelength dependence of attenuation, and on the changes that arise from the
transition from homogeneous to clumpy dust distributions in different star/dust
geometries. Embedded dust, especially when clumpy, leads to saturation at
fairly low reddening values with correspondingly gray attenuation functions.
This makes the assessment of the attenuation of the far-UV flux from starburst
galaxies difficult, if only UV/visual/near-IR data are available. Existing data
for UV-selected starburst galaxies indicate a range of UV attenuation factors
of 0-150. Our models reproduce the "Calzetti Attenuation Law", provided one
adopts SMC-type dust and a clumpy shell-type dust distribution surrounding the
starbursts. The average far-UV attenuation factor for the Calzetti sample is
found to be 7.4. The only relatively reliable measure for the UV attenuation
factor for single galaxies was found in the ratio of the integrated far-IR flux
to the far-UV flux, measured near 1600 A, requiring the measurement of the
entire spectral energy distribution of galaxies.
