This chapter reviews experimental data on giant magnetoresistance (GMR) in magnetic multilayers 150 151composed of ferromagnetic (F) and nonmagnetic (N) metals. Since Chapter 5 reviews the theory underlying GMR [1], theoretical issues in this chapter are treated only as needed to motivate and understand experimental data. As background, the review begins with a brief overview of the MR for a single magnetic film—called anisotropic magnetoresistance (AMR). It then covers, in more detail, the traditional current-in-plane (CIP) MR and the current-perpendicular-to-plane (CPP) MR in multilayers. Results on current-at-an-angle-to-the-plane (CAP) MR in multilayers [2, 3] and MR in granular films (GMR) [4–6] are treated only briefly. Figure 4.1a shows the different current directions for CIP and CPP. In a real CIP film, the aspect ratio in Figure 4.1a is inverted—the length of the CIP film in the direction of the current flow is much greater than the total thickness of its layers.152 Sample geometries: The magnetic field is applied horizontally (in-plane). (a) A multilayer composed of alternating ferromagnetic (<italic>F</italic>) and nonmagnetic (<italic>N</italic>) metals, and <italic>F</italic>-layer moment orientations for the <italic>P</italic>-state (all solid arrows pointing to the right) and <italic>AP</italic>-state (all dotted arrows pointing to the left). The arrows outside the multilayer show the current directions for CIP and CPP geometries. (b) A trilayer composed of a soft <italic>F</italic>-layer, the moment of which reverses at low field, and a hard <italic>F</italic>-layer, the moment of which stays fixed until a much higher field. (c) An EBSV, where an antiferromagnet (<italic>AF</italic>) pins the moment of an adjacent <italic>F</italic>-layer, leaving the other “free” <italic>F</italic>-layer to reverse its magnetization at low fields. (d) A granular <italic>N</italic> alloy, containing <italic>F</italic>-inclusions with initially randomly aligned moments that align in a large enough magnetic field. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429423079/4a422813-2b6a-402d-8595-9e3fe612bd48/content/fig4_1_PB.tif"/> https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429423079/4a422813-2b6a-402d-8595-9e3fe612bd48/content/fig4_1_OB.tif"/>