•D:  Crucial.  Strong dependence on baryon density.  Measured from interstellar absorption in UV/Far-UV with HST and FUSE.  Older values are [D/H] = 1.5 +/- 0.2 x 10-5, see text, but FUSE has more up to date results.  Keep in mind that stars destroy D, so these numbers are lower limits.  Can also look in the IGM using high-z quasars.