It is shown that the small-scale filaments generated by laser pulses in nonlinear media are the natural consequence of moving foci. Results of many controlled experiments performed with single-mode Q-switched laser pulses agree very well with the predictions of the moving focus model. The earlier experimental results on filaments reported in the literature are also shown to be consistent with the moving focus model. In the picosecond case, this model appropriately modified, gives essentially the same description as the so-called dynamic trapping model, which explains qualitatively the observed filament characteristics in that limit. Copyright © 1973 by The Institute of Electrical and Electronics Engineers, Inc.