Modern pewter, such as you might find in your kitchen, is primarily tin, alloyed with antimony and copper. The usual mixture is 91% tin, 7.5% antimony and 1.5% copper.
Long ago, pewter was made with lead, which contributed to lead poisoning in the populus, but this practice has long since passed out of use. Modern pewter is lead-free.
Be sure to care for your pewter properly.
References:
http://www.shirleypewter.com/history.html