The conventional x-ray image intensifier (XRII) has developed from an aid for fluoroscopy, where resolution is poor and noise high, through XRII photofluorography, where high resolution is needed and noise is moderate, to digital subtraction angiography, where resolution is moderate, noise must be very low and imaging capability must be pushed to its fundamental limits. These limits are not well understood for XRII. A better knowledge of the fundamental properties of the XRII will enable us to understand and determine these limits. We have measured detective quantum efficiency (DQE), spatial modulation transfer function (MTF), spatial noise spectra and temporal frequency response using monoenergetic x-rays derived from x-ray fluorescence. The results can be accounted for by an analysis based on the physical structure and operation of the XRII. Detailed measurements on every tube are unnecessary provided that the salient parameters (thickness, separation, and radii of curvature of the CsI phosphor layer, substrate and input window, MTF and temporal frequency response) are given by the manufacturer. With this fundamental knowledge it becomes possible to understand the limits of existing XRII and to explore the possibility of improving the images by digital enhancement, by better choice of incident x-ray spectra and by modifications of the XRII themselves. This will be illustrated by application to the case of digital subtraction angiography.