Proton-enhanced nuclear induction spectroscopy, also called cross-polarisation (CP), is a nuclear magnetic resonance technique invented by Michael Gibby and Alexander Pines while they were graduate students in the lab of Professor John S. Waugh at the Massachusetts Institute of Technology.

PENIS was one of the first of Pines’ experiments transferring spin orientation from one atomic nucleus to another, which has been one of the running themes throughout his career as a leading pioneer in the applications of NMR to the study of non-liquid samples. The PENIS technique was patented in 1972.