"In-cell nuclear magnetic resonance (NMR)" is a unique method for characterization of conformation, interaction and dynamics of proteins inside living cells at atomic level. Since the method was proposed by Dötch and co-workers in 2001 [1], its application had been limited to bacterial cells and oocytes of Xenopus laevis [2]. Recently, we reported a method for efficient delivery of (15)N-labeled proteins into human HeLa cells using cell-penetrating peptides, and measured high-resolution two-dimensional (1)H-(15)N correlation spectra of proteins in the cells. The in-cell NMR spectroscopy in human cells is capable of analyzing structures, interactions, dynamics and stability of proteins inside cells. Of its possible applications, we propose that in-cell NMR spectroscopy can be utilized as an effective step in protein-targeted drug development process, by demonstrating that interaction of FKBP12 with immunosuppressants administered extracellularly was successfully observed in living cells. This observation suggests that drug delivery and capability of target proteins inside cells for interaction with drugs can be investigated by in-cell NMR spectroscopy. More recently, an alternative way for intracellular delivery of labeled proteins for in-cell NMR was reported on 293F cells by Shimada and co-workers. Here, we review recent technical developments of in-cell NMR spectroscopy, and discuss potential usefulness for protein chemistry and drug screening process.