Quantitative estimates of delay distortion due to mode conversion in a multimode medium are made using an analysis based on modes coupled through power-transfer coefficients. This results in a simple translation from the spatial distribution of mode conversion to delay distortion without an intermediate step in the frequency domain. The expected value of the reconversion magnitude and its delay distribution relative to a driving impulse are found for (i) the case where the undesired mode loss is distributed (helix damped modes and higher-order circular electric modes) and (ii) the case where discrete mode filters are inserted (as in smooth-walled waveguide). Numerical estimates are given for TE 01 in 2-inch I.D. guides at 55 kmc. For both cases the power in the reconversion echo varies directly as the system length, and the shape of the echo is independent of length. For the case of distributed undesired mode loss the echo to impulse-excitation has an exponential shape in relative delay τ, varying as e−τ/τ 0, and for the case of partially absorbing mode filters the echo is a line-segment approximation to an exponential in τ (Fig. 4). The characteristic delay constant τ 0 is about 0.035 nanosecond for helix damped modes in an all-helix line, and is about 0.106 microsecond for TE 02 in either helix or smooth-walled guide. For solid-walled guide with mode filters every 300 feet the characteristic delay constant (similar to τ 0 ) is about 2 nanoseconds. Estimates are made for signal interference effects from such echoes, taking account of the fact that the most limiting requirements on echoes in some system arrangements occur at τ ≫ τ 0 , where the reconversion power is small. For PCM in smooth-walled copper waveguide with mode filters every 300 or 150 feel, it is concluded that pulse rates of 200 or 400 megabits might be used, with up to 20 or 40 miles respectively between regenerators; beat wavelength straightness variation mode conversion is controlling. for PCM in an all-helix waveguide, it is confluded that a pulse rate up to 5000 megabits and up to 746 miles between regenerators is permitted by mode conversion effects; diameter variations (TE02 conversion) are controlling. For transmission of frequency division multiplex multiplex via a frequency-modulated carrier (FDM-FM), estimates based on the discrete-echo theory of Bennett, Curtis and Rice suggest that 4000-mile transmission of 2000 channel groups is possible in all-helix waveguide; diameter variations are controlling. An rms frequency deviations (σ) increase the allowed system length (z) according to z ≈ (σ)2.8 Even in solid-walled waveguide there is a good prospect for 4000-mile FDM-FM using guide toleraces already achievable. Separate consideration is being given to delay distortion due to wave-guide cutoff dispersion, which will be appreciable in some configurations described and will require equalization.