One of the most spectacular examples of nanomagnetic systems is that of devices based on the giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) phenomena. The device response depends critically oil parameters such as layer thickness and chemical abruptness of the interfaces between layers.. which are nanometre-scale in thickness. We have used high resolution electron microscopy (HREM) and TEM chemical mapping to understand the microstructural origins of the magnetic and transport properties of magnetoresistive structures. We have also used Lorentz TEM and in situ magnetising experiments to analyse their magnetic structure and magnetisation reversal processes. (c) 2005 Elsevier B.V. All rights reserved.