Earth is a thriving planet orbitally sandwiched between the apparently lifeless worlds of Mars and Venus. NASA has historically focused on Mars, while the Soviet Union—as Scientific American's Amy Shira Teitel explains—focused its attention on Venus:
Exobiology primarily looks for life in three places: in underground oceans (where we may find life on Enceladus, Europa, or Ganymede), on the surface (where—so far—most of the life we've discovered on Earth resides), and in the atmosphere. Most of what I've written here deals with the first two categories, but in Venus' case there is a very good chance that we'll find atmospheric microbes and a greater-than-zero chance that we'll find atmospheric multicellular life. Because Venus' atmosphere is ludicrously thick, the prospect of an atmospheric ecosystem is not too far-fetched.
If Venus doesn't have atmospheric life, that will greatly reduce the odds that it has life at all. The average surface temperature on Venus is a hellish 864°F/462°C, and (so far) we've found no evidence of underground oceans.
Come late June or early July, the European Space Agency (ESA)'s Venus Express will plunge into the Venusian atmosphere and send feedback on its findings to Earth. What it learns—and what we learn from what it learns—may tell us if the possibility of atmospheric life on Venus is something we should more thoroughly investigate.