[en] In blazar research the key to answering such questions as 'What is the origin and emission mechanism of the high energy photons?' are the time lags between different wavebands. Progress towards measuring these lags has been hampered by the difficulty of obtaining longterm high energy monitoring, but this situation changed with the launch of RXTE. We 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 wavebands. Highlights include simultaneous flares in the X-ray/γ-ray and X-ray/IR, both new ground breaking results