The concept of multiscale modeling embodies the idea that a comprehensive description of a material will require an understanding over multiple time and length scales. A multiscale model requires that descriptions at all levels be consistent with each other, which can be particularly demanding for advanced materials and complex fluids. For crystalline materials, emerging modeling approaches have married smalland intermediate-scale descriptions in a highly effective manner, but challenges remain at long time and length scales. For soft materials, such as polymers or liquid crystals, modeling techniques have adopted a more or less systematic coarse-graining approach, in which atomic and molecular details are gradually blurred as one seeks to describe longer length scales. This approach presents its own brand of challenges. And, in spite of rapid advances, entire classes of materials, including amorphous glasses, foams, and gels, have resisted attempts to describe their structure and dynamics over long and relevant length and time scales. This issue of MRS Bulletin covers some areas of materials modeling in which enormous advances have been made, but which continue to raise intriguing questions and formidable challenges.