Cholesterol  is   often  found  distributed   nonrandomly  in  domains in  biological  and model membranes and has    been reported to be distributed     heterogeneously     among various intracellular membranes. While a large body of  literature  exists  on  the  organization   of  cholesterol  in   plasma membranes or membranes  with  high cholesterol  content,  very little  is known about   organization  of cholesterol   in  membranes  containing  low  amounts    of  cholesterol.     We   have previously  shown  using  a  fluorescent   cholesterol  analogue   (25-NBD-cholesterol)   that   cholesterol   may   exhibit local  organization  even  at  very   low concentrations  in membranes  as  evidenced   by  the   presence of  transbilay er   tail-  to  -tail   dimers  (Mukherjee  and Chattopadhyay (1996) Biochemistry).    We  have  further  investigated   the   role  of membrane   curvature and   thickness   on  transbilayer dimer arrangement   of  cholesterol  using  NBD-cholesterol.   We  find that  dimerization is not  favored in membranes  with  high curvature.   However, cholesterol  dimers  are  observed  again  if  the   curvature  stress  is  relieved.   In  addition,  we  have monitored   the  effect  of  membrane  thickness   on   the  dimerization   process.    Our   results   show that   the   dimerization   process  is  stringently controlled by  a   narrow   window   of   membrane thickness   (Rukmini  et  al.  (2001)  Biophys. J.).      We    also monitored    the     microenvironmental  features   of  cholesterol      monomers     and   dimers   using wavelength-selective  fluorescence.   The   environment  around   the  dimers  appears  to   be  more rigid (Mukherjee   and   Chattopadhyay  (2005)  Chem. Phys. Lipids).       We  have   recently  explored    the  lateral diffusion  of  the  monomer and  dimer   populations using  a  novel   wavelength-selective  FRAP  approach  (Pucadyil  et al.  (2007) J.  Phys. Chem. B). These results could be relevant in  membranes that have    very    low    cholesterol     content    such    as     the    endoplasmic     reticulum  and     the  inner mitochondrial membrane, and in trafficking and  sorting of cellular cholesterol.