The ""saucers"" and their varied cohorts are with us in spades as well as in space, but their mission is solely that of peace--and its preservation during the Earth's age of the atom. An anthology divided into two parts, this first takes up alien explorations of Earth as described by six noted s-f authors: Jack Williamson, Clifford D. Simak, Lester Del Ray, Eric Frank Russell, L. Sprague de Camp, and A. E. Van Vogt. Six equally well-seasoned star-gazers take over the ""Invasion"" section, in which power-mad earthlings are sometimes stopped short of total mayhem, at others are given sanctuary on still-intact planets after they have brought ours to a state of completed chaos. These writers are: Isaac Aslmov, John Beynon Harris, John Wyndham, Donald Wandrei, Clark Ashton Smith and Kendall Foster Crossen. David H. Keller and Nelson Bond provide the prologue and epilogue respectively of this slickly but sanely edited ""bem-mer"".