The parity conjecture is a weak version of Birch-Swinnerton-Dyer
Conjecture or more generally, Beilinson-Bloch-Kato Conjecture. It is
conjectured that the vanishing order of the L-function at the central
point has the same parity as the dimension of the Bloch-Kato Selmer
group. I will explain an approach to this conjecture for modular
forms by varying the modular forms in a p-adic family. This is a joint
work with Kiran Kedlaya and Jay Pottharst.