Reverberation mapping (RM) of active galactic nuclei (AGNs) has been used over the past three decades to determine AGN broad-line region (BLR) sizes and central black-hole masses, and their relations with the AGNs luminosity. Until recently the sample of objects with RM data was limited to low-luminosity AGNs ($L_{rm opt} lesssim 10^{46}$ ergs s$^{-1}$) and low redshifts ($z lesssim 0.5$). Here we present results from a reverberation-mapping project of some of the most luminous and highest redshift quasars that have been mapped to date. The study is based on almost twenty years of photometric monitoring of 11 quasars, six of which were monitored spectrophotometrically for 13 years. This is the longest reverberation-mapping project carried out so far on this type of AGNs. We successfully measure a time lag between the CIV$lambda$1549 broad emission line and the quasar continuum in three objects, and measure a CIII$lambda$1909 lag in one quasar. Together with recently published data on CIV reverberation mapping, the BLR size is found to scale as the square root of the UV luminosity over eight orders of magnitude in AGN luminosity. There is a significant scatter in the relation, part of which may be intrinsic to the AGNs. Although the CIV line is probably less well suited than Balmer lines for determination of the mass of the black hole, virial masses are tentatively computed and in spite of a large scatter we find that the mass of the black hole scales as the square root of the UV luminosity.
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