Introduction Conventional imaging relies predominantly on the use of magnitude images for information, whether it is T1- or T2-weighted imaging, or diffusion tensor imaging, for example. Apart from applications to flow imaging, phase information of the MR signal is usually discarded. The phase, however, contains useful information about local susceptibility differences between tissues.[1] Susceptibility-weighted imaging (SWI) is a means to enhance contrast in MRI based on tissue susceptibility differences, and it can use both phase and magnitude information of the MR signal.[2] In the past, phase images were difficult to interpret because they contained many artifacts from instrumental imperfections, background field inhomogeneities such as those resulting from air–tissue interfaces, and the main magnetic field itself. If these phase errors are corrected for, however, it is possible to combine the phase and the magnitude information in a way to create what is referred to as a susceptibility-weighted magnitude image. This triplet of images is becoming inceasingly used in clinical neuroimaging protocols. Data from SWI represent an adjunct to the information available with conventional spin density, T1- and T2-weighted imaging methods and complements other techniques discussed in this book, such as diffusion weighted imaging, perfusion-weighted imaging (PWI), and spectroscopic imaging. Applications for SWI range from visualizing blood products in tumors to measuring iron content in multiple sclerosis lesions.[1–85] In this chapter, we summarize the basic concepts behind SWI, the role of phase, the creation of high-resolution venograms, the quantification of iron, the combination of SWI, MR angiography, and PWI for a better understanding of flow and oxygen saturation, and discuss future directions.