  The type IIn SN 1998S is one of the most remarkable core-collapse supernovae
ever observed. It underwent a complex interaction with a substantial
circumstellar medium, resulting in radiation at wavelengths from radio to
X-rays. IR and optical observations have revealed a wide variety of broad and
narrow emission lines. Examination of the SN/CSM interaction and of the ejecta
spectra has allowed us to deduce that the supernova probably arose from a
massive, RSG progenitor having a large (>3200 AU radius), dusty circumstellar
disk. SN 1998S also developed one of the strongest, most persistent infrared
excesses ever seen in a supernova. IR/optical monitoring of SN 1998S has been
carried out to nearly 1200 days post-explosion. This includes coverage to
wavelengths as long as 4.7 microns, making SN 1998S only the second supernova
(after SN 1987A) to be observed in this spectral region. Fading of the central
and redshifted components of the late-time H I and He I line profiles suggests
strongly that dust condensed in the ejecta. However, it is less clear whether
the strong late-time IR emission arose from this dust, or from an IR echo in
the dusty CSM. One interesting possibility is that dust condensed in the cool
dense shell between the outer and reverse shocks, thus simultaneously producing
both the line obscuration and the IR emission.
