In blazar research the key to answering such questions as 'What is the
origin and emission mechanism of the high energy photons?' are the ti
me lags between different wavebands. Progress towards measuring these
lags has been hampered by the difficulty of obtaining longterm high en
ergy monitoring, but this situation changed with the launch of RXTE. W
e report on and discuss the results we have obtained from 18 months of
monitoring of 3C 279 and 3C 273 by RXTE in conjunction with other wav
ebands. Highlights include simultaneous flares in the X-ray/gamma-ray
and X-ray/IR, both new ground breaking results.