The paper gives methods for calculating the transient performance of the low-pass filter. It is shown that the currentImin the termination of anm-section filter whose critical frequency is ωc/(2π) is given byIm=1/ωc∫ωct0g(λ)E(ωct−λ)dλwhereE(ωct) is the voltage applied to the filter att=0, andg(ωct) is the response of the filter to a unit impulse. Results are given in the cases for whichg(λ) can be calculated, which include filters terminated in a resistance of □(L/C), and short- and open-circuits (in this last case the voltage across the open-circuit is found). The method also gives solutions for the uniformly dissipative filter with these terminations. For all these casesg(λ) is expressed in terms of the repeated integrals of Bessel functions, andImcan be found in terms of these functions by, at most, one numerical integration of the above equation. These solutions prove very suitable for numerical computation, especially at the start of the transient.The method has been applied to work out in detail the transient performance of 3-, 4-, and 5-section filters terminated in a resistance of □(L/C). The concept of delay time, based on a treatment using “ideal” filters or “smooth” lines, is examined and shown to be very approximate. It is pointed out that the solutions obtained can be interpreted in terms of waves propagated to and fro through the filter, but that this is an approximate conception, for the “wave” starts to arrive immediately any voltage is applied.