The Arecibo Legacy Fast ALFA Survey has been designed to cover 7000 square degrees of sky at L band. The HI line will thus be surveyed between 1340 and 1430 MHz, with a resolution of 25 kHz. Observations are conducted in drift mode, and each region of sky will be visited twice. Observations started in early February 2005. A precursor run was carried out in the second half of 2004. The scientific motivation, goals and expectations of the survey will be presented, as well as examples of data products and the science results of the early observations.