Claudio Pellegrini ( May 9, 1935) is an Italian/American physics and emeritus professor at University of California, Los Angeles (UCLA), known for his pioneering work on X-ray free electron lasers and collective effects in relativistic particle beams.

In the early 1960s, he was at the Nordic Institute for Theoretical Physics (NORDITA) in Copenhagen, working on an alternative formulation of the theory of general relativity using tetrad fields to obtain, among other things, a better description of the energy-momentum complex.

At Brookhaven, he studied free electron lasers (FELs) and their application to the generation of high intensity coherent X-ray pulses.

[7][8][9] This work and the 1992 proposal led to the construction of the Linac Coherent Light Source (LCLS), the first 1-angstrom X-ray laser, which has been successfully operating at SLAC since 2009.

LCLS has opened a new window for the exploration of atomic and molecular science at the one angstrom-one femtosecond length and time scale characteristic of these phenomena.