Leading logarithms in field theory

We consider the Sudakov form factor in effective theories and we show that one can derive correctly the double logarithms of the original, high-energy , theory. We show that in effective theories it is possible to separate exp licitly soft and hard dynamics, these being two regimes related to velocity conserving and to velocity changing operators respectively. A new effectiv e theory is sketched which extracts the leading collinear singularities of the full theory amplitudes. Finally, we show how all leading logarithmic ef fects in field theory can be obtained by means of simple effective theories , where they correspond to a renormalization effect.