Interactions between the plasmons of noble metal nanocrystals and organic molecules can give rise to a number of interesting phenomena, which have recently received enormous interests and will lead to a wide range of technological applications. The interaction behavior is strongly dependent on the electronic structures of organic molecules. Here we review our recent work on the three types of interactions between Au nanocrystals and organic molecules. The first is the interaction between plasmons and molecules without absorption, which leads to refractive index change-based plasmonic sensing. The refractive index-induced plasmon resonance shifts are found to be strongly dependent on the plasmonic properties of Au nanocrystals. The second is the plasmonic molecular resonance coupling, where molecules exhibit strong light absorption around the plasmon resonance energies. The strong resonance coupling gives hybridized states and anti-crossing behaviors. The third occurs when molecules possess both absorption and fluorescence emission around the plasmon energies. The fluorescence intensities of fluorophores can be enhanced remarkably in the presence of Au nanocrystals. (C) 2010 Elsevier Ltd. All rights reserved.