This book is a compilation of papers presented at Symposium 4, Materials Characterization, held at the XVIII International Materials Research Congress 2009 in Cancun, Mexico. The symposium was co-organized by the Sociedad Mexicana de Materiales (MRS-Mexico) and the Materials Research Society (MRS). These proceedings offer materials scientists from around the world a common platform to exchange their findings and discuss developments, and begin collaborations at the national and international levels. Topics include: materials characterization by X-ray diffraction, infrared spectroscopy, scanning electron microscopy and transmission electron microscopy; characterization of pipeline steels used in the oil industry; characterization of nanostructured materials; microstructural characterization of welding and joining of materials; materials characterization for biomedical applications; materials characterization for industrial applications; materials characterization by photoacoustic and photoluminescence techniques; and microstructural characterization of stainless steels.