The concept of interplanetary transfer of life requires that organisms cope with three major challenges: (1) The escape process, (2) the long-term stay in space, and (3) the landing process. The first step involves hypervelocity impact by comets or asteroids under strong or moderate shock metamorphism of the ejected microbe-bearing rock fragment. Experiments have shown that bacterial spores can survive such a simulated meteorite impact. The second step deals with the ability of microorganisms to withstand the complex interplay of the parameters of space, e.g.: vacuum, ultraviolet and ionizing radiation, and temperature extremes, when traveling in space over extended periods of time. Experiments in space, such as on board of Apollo, Spacelab 1 (SL 1), the Long Duration Exposure Facility (LDEF), FOTON, MIR, and the European Retrievable Carrier (EURECA), as well as at space simulation facilities on ground, have given the following five results: (1) Extraterrestrial solar UV radiation is a thousand times more efficient than UV at the surface of the Earth and kills 99% of bacterial spores within a few seconds; (2) space vacuum increases the UV sensitivity of the spores; (3) although spores survive extended periods of time in space vacuum (up to 6 yr) genetic changes occur, such as increased mutation rates; (4) after 6 yr in space, up to 70% of bacterial spores survived if protected against solar UV radiation and dehydration; (5) spores could escape a hit of a cosmic HZE particle, e.g.: iron ion, for up to 1 Ma. Calculations using radiative transfer models for cosmic rays and biological data from accelerator experiments have shown that a meteorite layer of 1 m or more effectively protects bacterial spores against galactic cosmic radiation for 1 Ma or more. It is concluded that radiation-resistant microbes could survive a journey from one planet to another in our solar system if they are located inside a meteorite thus shielded against cosmic radiation. However, viable transport between solar systems seems to be not possible, assuming impact ejection as the first step. The last step, capture and landing on a planet depends very much on the atmospheric conditions and the size of the meteorite. So far, few experiments have been done to investigate the effects of the landing process.