The present invention relates to novel mutant thioesterase enzymes and naturally-occurring equivalents thereof, compositions made from such enzymes and uses of thioesterase enzymes. In particular, the present invention provides mutant thioesterase enzymes that have altered properties, for example, altered substrate specificity, altered activity, altered selectivity, and/or altered proportional yields in the product mixtures. The present invention also provides polynucleotides encoding such mutant thioesterase enzymes, and vectors and host cells comprising such polynucleotides. The invention further provides for novel uses of thioesterases in the production of various fatty acid derivatives, which are useful as, or as components of, industrial chemicals and fuels.