[en] The design of large e⁺e^- storage rings at present looks very conventional. A natural thought is that the building of a large storage ring will be mainly a question of size and cost. This is indeed true for LEP. However, very stringent theoretical and technical difficulties arise which show that energies far beyond the present LEP value will be difficult to achieve (although it is still hard to set a precise limit) unless new accelerator techniques are shown to work. The present report deals with some of these difficulties and makes an extensive use of scaling laws. However, the first aim has been to introduce non-experts to the basic principles which enter in the design of e⁺e^- storage rings. (Auth.)