ELT science drivers stress aperture, Strehl ratio, PSF definition and stability, field of view, wavelength range, flexibility, low polarisation and thermal emittance, auxiliary instruments, site and infrastructure. Applicable science categories are planets and planetary systems, stars and stellar systems, galaxies and galaxy clusters, and cosmology. ELT observations are needed for our own and other planetary systems. The study of planetary disks and formation requires ELT data. ELT results, with emphasis on PSF quality and stability, are crucial to the search for earth-like planets, especially those favourable for life. Investigation of star formation and stellar evolution requires ELT performance, as does the study of final stellar stages. ELTs are necessary for extremely high time resolution, details of stellar surfaces and astroseismology. Galaxy formation studies will benefit dramatically from ELT data, as will studies of large-scale development of galaxies and galaxy clusters over cosmological time scales. Detecting active galactic nuclei requires ELTs. ELT data are crucial for examining the structure and evolution of the universe. Observations of supernovae and other standard sources over very large distances are necessary for mapping the expansion of the universe and determining its acceleration or deceleration. Comparisons of Euro50 with VLTs and HST show a dramatic gain. The complementarity of Euro50, NGST and ALMA is noted.