The mouse has become one of the most popular mammals for biomedical research. With the growing amount of experimental and genetic mouse models of human disease, there is a need for efficient and reliable methods for characterizing abnormalities in mouse anatomy and physiology. Common tumour imaging modalities for use with small animals include magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), single photon emission computed tomography (SPECT), bioluminescent imaging and ultrasound. Each of these modalities has its advantages and limitations, which will be briefly outlined here. The use of MRI provides a promising technology providing informative and meaningful measures in a variety of mouse models, and several current MR imaging techniques of small animals will be presented. Differences in contrast in soft tissues depend on endogenous differences in water content, relaxation times and diffusion characteristics of the tissue. The specificity of MRI can be further increased by exogenous contrast agents, such as gadolinium chelates, which have been successfully used for imaging of hemodynamic parameters including blood perfusion and vascular permeability. Cellular MR imaging for visualising targeted cells in living organisms are also performed, using cell labelling with superparamagnetic iron oxide particles. However, due to the low sensitivity of MRI, compared with nuclear imaging, high local concentrations at the target site are required to generate detectable MR contrast.