ELSI provides and enhances scalable, open-source software library solutions for electronic structure calculations in materials science, condensed matter physics, chemistry, molecular biochemistry, and many other fields. ELSI focuses on methods that solve or circumvent the Kohn-Sham eigenvalue problem in density-functional theory. The ELSI infrastructure should also be useful for other challenging eigenvalue problems. One of the key design pillars of ELSI is portability and support for various computing environments, from laptop-type computers all the way to the most efficient massively parallel supercomputers and new architectures (GPU and manycore processors). The libraries supported in ELSI are: ELPA (massively parallel dense eigensolvers); libOMM (orbital minimization method); PEXSI (pole expansion and selected inversion); EigenExa (massively parallel dense eigensolvers); SLEPc-SIPs (sparse eigensolver based on shift-and-invert spectral transformations); NTPoly (density matrix purification)