The front cover of this month's issue has two clear, yet almost opposite, messages that relate to the theme of signalling. On the one hand, we can see that the basic purpose of signalling is to communicate; in this case, the simple practice of semaphore — holding two flags in certain positions — serves effectively to transfer messages. On the other hand, the message that is spelled out — 'the complexity of signalling' — indicates how basic systems can evolve rapidly, with increasing intricacy.

With this in mind, this month's issue is focused around a central question: how do signals that are received at the cell membrane reach the nucleus? We are offering a poster free with this issue and available free online throughout September (at http://www.nature.com/reviews/molcellbio/) to provide a conceptual illustration of the mechanisms by which this can occur. You can also access further signalling-related articles at our 'Signalling: from surface to centre' Web Focus site.

This issue gives an overview of several well-established signalling pathways, from those induced by seven-transmembrane receptors (see the article on page 639 by Kristen L. Pierce, Richard T. Premont and Robert J. Lefkowitz) through phosphorelay systems (for example, mixed-lineage kinases, described on page 663 by Kathleen A. Gallo and Gary L. Johnson) to nuclear-receptor-mediated responses (see the Timeline article on page 702 by Jamshed R. Tata).

More recently, however, new mechanisms for signal transduction have emerged (see the article by Mark E. Fortini on page 673, and the Opinion by Alan Wells and Ulrich Marti on page 697). It is not surprising, therefore, that the signalling field often seems to be overwhelming, and our aim this month is to make it accessible to everyone.