Mobile web design

1

so, ive been out of the website design game for a few years but recently my brother asked if i could revamp his site to flash. of course i agreed, but with all the internet friendly mobile devices out now, should i be worried about the site being compatible with them? we haven’t discussed that area yet but i would like to be ready with some answers when we do. i know i can always google it to find info but i wanted to check here first, you all have always been such a big help.