The adaptation of therapies to the specificity of each tumour increases the need of experimental models to perform the first evaluation of a drug or a biological agent. Significant progress has been made during the last 10 years in the domain of imaging of (mostly murine) tumour models. It is therefore possible to address questions relevant to tumour treatment and to the roles of individual molecular pathways in ways that cannot be addressed directly in human subjects. The development of new imaging techniques and of new tracers or reporters enables specific, highly sensitive and quantitative measurement of a wide range of tumour-related parameters:In vivo imaging gives access to precise measurements of tumour size, number and growth, as illustrated by Jouannot with high-frequency ultrasound imaging, and by De Clerk with micro-CT. This may help the longitudinal follow up before and after therapy. Vascular development is a key factor of tumour growth. To qualify anti-angiogenic therapies, quantification of tumour angiogenesis and blood flow is needed. Ultrasound imaging, helped by contrast agents, is now settling as a basic low-cost technique, as illustrated by Lucidarme. Fluorescence molecular tomography also gives a global appreciation of vascularity in superficial tumours, as illustrated by Montet. The ultimate visualization at microvessel scale is obtained by in vivo confocal microscopy applied to small tumours growing in a skinfold chamber, as illustrated by Jain. CT and MRI can be applied to the characterization of the vascular network in larger tumours and the surrounding organs. The reports by Cuenod show the possibility to distinguish arterial and venous vascularizations of liver lesions by func-tional CT and to quantify tumour angiogenesis by dynamic contrast-enhanced MRI. The biochemical characterization of tumours is now helped by imaging probes that selectively accumulate in tumours or that become activated by tumour-specific molecules in vivo, as illustrated by Law et al.Other tumour imaging strategies have been developed that rely upon the detection of reporter transgene expression in vivo, and these too have made a significant impact on both the versatility and the specificity of tumour imaging in living mice. Optical imaging of apoptosis can be performed by using NIR fluorescent probe activatable by a caspase or by using a dye coupled to annexin, labelling phosphatidylserine residues, as illustrated by the reports by Schellenberger et al., and by Law and Tung. Similar studies, adaptable to larger lesions, are available through SPECT imaging as shown by Del Vecchio.These advances in tumour imaging are set to have a profound impact on our basic understanding of in vivo tumour biology and will radically alter the application of mouse tumour models in the laboratory.

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