Recent advances in light microscopy have allowed researchers to directly visualize immune cells in situ as they migrate from one site to another, invade infected tissues and engulf pathogens. It also has allowed the visualization of interactions that occur between various cell types in lymph nodes and at peripheral sites in living animals. These high-resolution images have enabled us to broaden our understanding of how, where and when cells interact during the initiation of an immune response and of the dynamics of immune-cell behaviour.

In their Review on page 497, Ronald Germain and colleagues discuss the tools and techniques involved in imaging that have emerged over the past few years, including technical details of single-photon and two-photon imaging, fluorescent-labelling techniques and alternative methods of imaging. They also provide a detailed analysis of the advantages, disadvantages and limitations of the methodology that is currently in use.

Indeed, such techniques have been used to obtain snapshots that capture the early beginnings of development of the fetal thymus, with the initial colonizing cells depicted, as featured in the Opinion article by Eric Jenkinson and colleagues on page 551. These studies, together with new cell-isolation techniques, led the authors to challenge the current dogma regarding the capacity of colonizing progenitors to generate both T cells and B cells, and the role of Notch signalling in determining lineage choice.

The use of high-resolution imaging, however, is not without its drawbacks, and Germain and colleagues discuss potential artefacts that occur in such experiments and how these can be recognized and avoided. This Review is a useful guide for those thinking of adopting such imaging experiments for their own research.