Type IIn supernovae (SN IIn) is highly diverse and badly understood class with a
common feature: they show narrow emission lines indicative of a dense circumstellar
matter (CSM). The narrow emission lines could originate either from the undisturbed
circumstellar gas excited by 
X-ray/ultraviolet radiation or from shocked circumstellar clouds. In most SNe IIn the
total luminosity is fully powered by the SN/CSM interaction. The origin of the dense
CSM is likely very much different and could be related either with a heavy wind or
with a violent ejection of massive shell. The wide range of the ambient density is
manifested in enormous range of SNe IIn luminosity: from -14 mag (2008S) to -22.5 mag
(2008es). The current comprehension of physics behind the light curves and spectra of
different SNe IIn is reviewed and challenging problems that impede our understanding
of this category of SNe are emphasised.