 Harriet Taylor Mill (1807–1858) poses a unique set of problems for an encyclopedist. The usual approach to writing an entry on a historical figure, namely presenting a straightforward summary of their major works and then offering a few words of appraisal, is not available. This is because she worked in such close collaboration with John Stuart Mill that it is exceedingly difficult to disentangle their contributions to the products of their joint effort. In attempting to assess Taylor Mill’s philosophical career, one encounters sharply conflicting reports about her ability from people who knew her, contradictory evidence about her role in authoring several books and essays, and widely varying judgments about how much influence she exerted on Mill’s thought and work.