It is clear that knowledge of the relative phases is essential if we wish to find the atoms in a crystal. So what do we do if we do not have phase information? ‘Seeing atoms’ describes the phase problem and the different methods of phase determination used by crystallographers: a difference Fourier map, the Patterson method, electron density maps, multiple isomorphous replacement, multiple-wavelength anomalous dispersion using synchrotron radiation, which is often used in macromolecular crystallography, molecular replacement, commonly used in protein crystallography, the Sayre equation, a mathematical relationship that enables probable values for the phases of some diffracted beams to be found, and a new technique called charge flipping. Access to the complete content on Very Short Introductions online requires a subscription or purchase. Public users are able to search the site and view the abstracts and keywords for each book and chapter without a subscription. If you have purchased a print title that contains an access token, please see the token for information about how to register your code. For questions on access or troubleshooting, please check our FAQs, and if you can''t find the answer there, please contact us. Source.