Recent developments in Coupled-Cluster (CC) theory of excited states, which allow the application of these expensive methods for nucleobases and even for their complexes, are overviewed.In particular, ways of including triple excitations are investigated in detail.It was found that the EOM-CCSD(T) method represents an economic and accurate compromise.Therefore we used this method to investigate nucleobases in biological environment: effect of hydration, that of the attached sugar, as well as pairing in Watson-Crick or stacked form on the excitation energies have been investigated.