Interactions and correlations in condensed matter systems often manifest themselves in striking and novel properties of the materials. Many examples can be found among superconductors and superfluids, quantum magnets, integer and fractional quantum Hall systems. In spite of the apparent differences among various materials and compounds, there are numerous universal phenomena that take place in interacting fermionic and bosonic systems. The main focus of Eugene Demler's work has been developing general theoretical tools for understanding the effects of interactions, and establishing a common framework for understanding the physics of strongly correlated systems. Demler's research has addressed various properties of high temperature superconductors, heavy fermion and organic superconductors, quantum Hall systems, and quantum antiferromagnets. Demler's research interests also include mesoscopic superconductivity, magnetic and superconducting proximity effects, understanding the effect of dissipation on quantum phase transitions, and Bose-Einstein condensation of alkali atoms.