Contents: •9 • Introduction (Tomorrow's Worlds) • essay by Robert Silverberg •13 • Sunrise on Mercury • (1957) • shortstory by Robert Silverberg [as by Calvin M. Knox ] •29 • Before Eden • (1961) • shortstory by Arthur C. Clarke •41 • Seeds of the Dusk • (1938) • novelette by Raymond Z. Gallun •71 • The Black Pits of Luna • (1948) • shortstory by Robert A. Heinlein •85 • Crucifixus Etiam • (1953) • shortstory by Walter M. Miller, Jr. •107 • Desertion • [City] • (1944) • shortstory by Clifford D. Simak •121 • Pressure • [Matter Transmitter] • (1969) • novelette by Harry Harrison •139 • The Planet of Doubt • [Ham Hammond] • (1935) • novelette by Stanley G. Weinbaum •169 • One Sunday in Neptune • shortstory by Alexei Panshin •179 • Wait It Out • [Known Space] • (1968) • shortstory by Larry Niven