Many have questioned the reliability and accuracy of Wikipedia. Here a different issue, but one closely related: how broad is the coverage of Wikipedia? Differences in the interests and attention of Wikipedia’s editors mean that some areas, in the traditional sciences, for example, are better covered than others. Two approaches to measuring this coverage are presented. The first maps the distribution of topics on Wikipedia to the distribution of books published. The second compares the distribution of topics in three established, field-specific academic encyclopedias to the articles found in Wikipedia. Unlike the top-down construction of traditional encyclopedias, Wikipedia’s topical coverage is driven by the interests of its users, and as a result, the reliability and completeness of Wikipedia is likely to be different depending on the subject-area of the article.