TUNA (Towards a UNified Algorithm for the generation of referring expressions) is a research project funded by the UK's Engineering and Physical Sciences Research Council (EPSRC). The TUNA Corpus of Referring Expressions is built with the contributions from 50 native or fluent speakers of English and it contains about 2000 descriptions (referring expressions). Participants described objects (targets) in visual domains by typing and submitting referring expressions that distingued them from other objects that were shown simultaneously (distractors): Each experimental trial consisted of one (singular) or two (plural) targets, plus six distractors. Each description is richly annotated with semantic information, including information about all the other objects that the human authors saw. The TUNA Corpus was annotated with the main objective to evaluate the output of algorithms for the Generation of Referring Expressions (GRE) with Natural Language Generation, with particular regard for the semantic content of the expressions.