What is NATBIB? It is a LaTeX (2e, with some support for 2.09) package to act as generalized interface for standard and non-standard bibliographic style files (BibTeX). It is intended primarily to produce author-year style citations from a variety of non-standard .bst outputs; it therefore replaces the special and individual packages that go along with such .bst files: apalike.sty for apalike.bst et al. newapa.sty newapa.bst, chicago.sty chicago.bst, named.bst et al. harvard.sty agsm.bst, dcu.bst, kluwer.bst et al. astron.sty astron.bst, apa.bst, humanbio.bst, et al. authordate.sty authordate.bst, aaai-named.bst The above .bst files may also be used with natbib to produce author-year or numerical citations, so that switching from author-year to numerical is only a matter of selecting an option. The standard .bst files (plain et al) and other numerical citation .bst files are also understood, but they can produce only numerical citations. Replacement .bst files plainnat.bst abbrvnat.bst unsrtnat.bst are provided to produce the same formatting of the reference list as the standard equivalent, but allowing both author-year and numerical citations. See my parallel custom-bib system for generating customized bst files. For installation instructions and more details, see README.vxx. Patrick W. Daly Max-Planck-Institut f\"ur Sonnensystemforschung D-37191 Katlenburg-Lindau, Germany E-mail: daly@mps.mpg.de