Throughout the 1990s, we have witnessed the burgeoning interest and rapidly advancing technologies in the" eld of information visualization (Card, Mackinlay & Shneiderman, 1999; Chen, 1999; Ware, 2000). This" eld has captured the imagination of researchers and practitioners from a wide range of disciplines all over the world. Increasingly powerful and visually appealing information visualization techniques are" nding their way into everyday life in our information society. The widespread proliferation of information visualization techniques has also highlighted the need for principles and methodologies for empirical evaluation of these techniques. Urgently needed are improved methods in areas such as task analysis, usability evaluation and usage analysis, especially those with special emphasis on visualization} intrinsic user interfaces. Until recently, research in information visualization has largely focused on the development of innovative computing techniques. George Robertson, the inventor of wellknown information visualization techniques such as Cone Trees (Robertson, Mackinlay & Card, 1991), was among the" rst to draw our attention to the importance of empirical evaluation with his keynote speech in 1998 at the IEEE Information Visualization Symposium. We are" nally beginning to see evaluations of advanced visualization techniques, but progress has been slow and isolated (eg. Pirolli, Card & Van Der Wege, 2000; Graham, Kennedy & Hand, 2000).

This special issue aims to provide a timely and unifying forum for researchers and practitioners to tackle some of the fundamental and practical issues concerning empirical evaluation of information visualizations. We have solicited nine original articles on topics related to this theme. Two thematic aspects become apparent in this special issue, namely, evaluating well-known information visualization techniques and analysing the use of information visualization techniques in a practical context. A detailed introduction will be given below to guide the reader of the special issue.