In its December 1990 fly-by of Earth, the Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum, and atmospheric methane in extreme thermodynamic disequilibrium; together, these are strongly suggestive of life on Earth. Moreover, the presence of narrowband, pulsed, amplitude-modulated radio transmission seems uniquely attributable to intelligence. These observations constitute a control experiment for the search for extraterrestrial life by modern interplanetary spacecraft -- as well as an indication of which types of observations may be fruitful in the search for life on planets of other stars. But mere detection of H₂O and CO₂ -- or even O₂/O₃ -- in the atmosphere of a terrestrial planet is insufficient evidence of life: Nonbiological photolytic processes can -- depending on atmospheric structure and oxidation state of the crust -- generate significant quantities of O₂.