"Tahini is a paste similar to peanut butter but made from sesame seeds. It has a unique and delicious flavor and can usually be found near the peanut butter or in the natural food aisle (or ordered online - I've ordered the Lior brand from Amazon). I've never substituted peanut butter so I can't attest to the results. Tahini is worth keeping on hand if you make (and consume) a lot of hummus like I do...."