Transmission line theory models distributed electrical parameters where the physical line length is comparable to or exceeds the signal wavelength, rendering lumped circuit approximations invalid. Governed by the Telegrapher's equations, voltage and current propagate as electromagnetic waves with characteristic impedance Z₀ and finite propagation velocity v_p. Any impedance mismatch between the line and the terminating load creates reflected waves, producing standing wave patterns characterized by the Voltage Standing Wave Ratio (VSWR).