ACHyear the IEEE Visualization Conference continues to be the premiere forum for presenting state-of-the-art research in all topics related to visualization. In this issue, we present revised and extended versions of some of the best papers from the 2004 edition of the conference. The selection process for inviting papers for this special issue followed the pattern set by the 2003 conference. In June 2004, after the conference paper review and selection was complete, the best papers were identified. The chairs considered the written comments from the reviewers as well as their own reading of the papers in this process. The authors of the papers identified were invited to prepare revised and extended versions of their conference papers during the summer and early fall of 2004. We then sent out the revised papers for a full journal-level review by external referees. The results of the revision and review process are the six papers which appear in this issue. The goal of visualization is to generate images that enable people to gain understanding. There are many facets to this endeavor, ranging from complex mathematical issues to issues of how we as humans interact with images. The papers included here span this range of activity. Two of the papers deal with the problem of interactive visualization of massive triangle models. The papers take different approaches to different versions of the problem. They have in common, though, introducing new algorithms based on new data representations that are demonstrated in fully implemented systems. In “Real-Time Optimal Adaptation for Planetary Geometry and Texture: 4-8 Tile Hierarchies,” Hwa, Duchaineau, and Joy consider the problem of terrain visualization. They introduce a unique diamond data structure to represent the mesh. The hierarchical data structure has the interesting property that diamonds in the hierarchy overlap with, but are not coincident with, their children. This structure facilitates more gradual transitions in level of detail than have been possible with previous approaches. The conference paper this is based on received the IEEE Visualization best paper award. “Quick-VDR: Out-of-Core View-Dependent Rendering of Gigantic Models” by Yoon, Salomon, Gayle, and Manocha uses a new representation for general triangle meshes—CHPM, a clustered hierarchy of progressive meshes. These clusters are the basic organization for level of detail management, culling, and out-of-core rendering. The system is demonstrated on a wide range of data sets, ranging from isosurfaces from turbulent flow simulations to data from 3D scanning of cultural heritage objects. Two of the papers focus on visualizing complex fields. Naive visualizations of these types of data sets result in overwhelming clutter. Both papers present robust methods for extracting key topological features to reduce massive data sets to elegantly simple representations revealing their basic structure. Theisel, Weinkauf, Hege, and Seidel consider temporally varying fields in “Topological Methods for 2D TimeDependent Vector Fields Based on Stream Lines and Path Lines.” The visualization of stream line topology has been previously studied. In this paper, the alternative of path line topology is introduced for the same class of data sets. The authors describe how to extract features in both approaches and compare the characteristics of the flow they reveal. Zheng, Parlett, and Pang consider tensor fields in