The Research Article "A bacterium that can grow by using arsenic instead of phosphorus" by F. Wolfe-Simon et al., published online 2 December 2010, was the subject of extensive discussion and criticism following its online publication. Science received a wide range of correspondence that raised specific concerns about the Research Article's methods and interpretations. Eight Technical Comments that represent the main concerns, as well as a Technical Response by Wolfe-Simon et al., are published online in Science Express at the addresses listed in this note. They have been peer-reviewed and revised according to Science's standard procedure.