The Viking mission was NASA's second project to Mars. In the 1960's, the Mariners 4, 6 and 9 made it to Mars. These missions returned images of a moon-like planet with an atmosphere consisting of mainly carbon dioxide and with polar ice caps also consisting of carbon dioxide.
In 1976, the Viking missions started. These were set with three biological and chemical experiments to look for metabolic products as well as to do measurements of gasses in the atmosphere and soil samples.
They found that there was a total absence of organic material. Any processes that produced similar by-products to organic processes can be explained by pure chemical processes.
New missions set for Mars are the Pathfinders. NASA is planning a series of Mars landings to explore the surface of Mars and to look for other fossils. The first mission was launched in December '96, and its mission is to scout for locations, such as defunct hot springs and other spots that may be good places to start looking for fossils. There are landings on Mars planned for every 2 years until 2006.
For more information on the Viking missions or future missions check out NASA's web page @: