  I provide a short review of the properties of Narrow-line Seyfert 1 (NLS1)
galaxies across the electromagnetic spectrum and of the models to explain them.
Their continuum and emission-line properties manifest one extreme form of
Seyfert activity. As such, NLS1 galaxies may hold important clues to the key
parameters that drive nuclear activity. Their high accretion rates close to the
Eddington rate provide new insight into accretion physics, their low black hole
masses and perhaps young ages allow us to address issues of black hole growth,
their strong optical FeII emission places strong constraints on FeII and
perhaps metal formation models and physical conditions in these emission-line
clouds, and their enhanced radio quiteness permits a fresh look at causes of
radio loudness and the radio-loud radio-quiet bimodality in AGN.
