Scientists have high hopes of findings signs of life on Mars (fossilized remnants of ancient microbes more likely than existing life). A lot of this hope is based on the evidences that Mars once had a lot of liquid water (and may still have some water locked up underground as ice which may occasionally melt and cause local flooding). It is already known that the north ice cap is water ice (the south ice cap is CO2 ice). There are other hints - - like what we see upon examination of Martian meteorites.
NASA's Viking mission in the 1970s ran 3 tests on Martian soil looking for existing microbial life. At first, the test results seemed positive...adding food (organic stuff) to the soil resulted in a reaction (like converting the organics to CO2). But in the end it was decided that a more likely explanation was that the reaction was a result of chemistry, not biology (e.g., Martian soil oxidizes organic materials). As Ivan said, some people still think the Viking results were positive, but they are in the minority.
So, most scientists think the surface of Mars is currently without life (e.g., any organic life would be oxidized...disintegrated...note also that Mars has a very thin atmosphere, extreme temperature variations, and no protective ozone layer). But there are still hopes for fossils of microbes from times past, and perhaps microbial life hanging on in the subsurface.