Characterisation of the ultra-short optical pulses produced by infrared ffee-electron lasers (FELs) is an important task, not only for the further development of free electron lasers and their theory, but also for their operation as a research tool. The setting up and optimisation of the FEL requires effective and reliable diagnostics tools. This thesis presents techniques for the measurement of sub-picosecond optical and electron pulses. A range of techniques is developed that allows measurements of the electric field of both optical pulses and electron bunches to be made with an accuracy of better than 100 fs. These techniques have been used to obtain the first complete electricfield characterisation of ultra-short pulses from a far-infrared FEL; to study the formation of singlesided exponential optical pulses in two FELs; and to obtain the longitudinal profile of electron bunches, both by probing the near-field transition radiation and by directly sensing the Coulomb field of the electron bunches. Although the techniques described are not truly single-shot - requiring measurements averaged over a period of a few microseconds - ways in which they could be extended to provide single-shot capability are discussed.