The VUV-FEL at DESY, Hamburg, will require novel techniques to characterize the longitudinal charge distribution of the electron bunches that drive the free-electron laser. Two reasons are the short bunch lengths that will be obtained by the bunch compression schemes and the high sensitivity of the lasing process to spikes in the charge distribution. This contribution will give an overview of the different measurement techniques to access bunch lengths on the order of 50 μ m. Approaches to reconstruct the bunch shape by Fourier analysis of the coherent part of the infrared spectrum of synchrotron, transition, diffraction and undulator radiation will be presented. Furthermore, methods working in the time domain, like the use of transverse mode cavities or the principle of electro-optical sampling, will be discussed. Examples from TTF2 will be used to show possible technical implementations of the corresponding techniques.