Web search engines face an extremely heterogeneous user population from web novices to highly skilled experts. Currently, the search strategies of the experienced web searchers are largely unknown and this paper addresses this issue by an observational study. Seven computer scientists were observed during and interviewed after performing their own work-related web search tasks. The observations indicated that experts have effective means for enhancing the searching, such as: using multiple search terms and operators, frequent query editing, using multiple windows, versatile result saving, and using the 'Find' functionality. On the other hand, even the experts had misconceptions about the default operator and the ordering of the returned documents. Based on the results we suggest that search user interfaces should: 1) suggest alternative search terms, 2) explain search operators in natural language, 3) provide search history, and 4) facilitate users’ orientat ion to the res...