Seismic inversion or tomography (tomo=slice+graph=picture) is a procedure for reconstructing the earth's velocity model from seismic data. There are at two main types of seismic data to be inverted: traveltime data and waveform data. Traveltime tomography reconstructs earth velocity models with several times lower resolution compared to waveform tomograms. But on the other hand traveltime tomography is typically much more robust, easier to implement, and computationally much cheaper. In this lecture I will present the traveltime tomographic method, discuss its capabilities and limitations, and illustrate its use with data examples. Some good reference books are Nolet (1987), Lines (1988) and Menke (1984).