  We present a transmission line theory of exceptional points of degeneracy
(EPD) in coupled-mode guiding structures, i.e., a theory that illustrates the
characteristics of coupled electromagnetic modes under a special dispersion
degeneracy condition, yet unexplored in the contest of gain and loss. We
demonstrate the concept of Parity-Time ($\cal{PT}$)-symmetry in coupled uniform
waveguides with balanced and symmetric gain and loss and how this condition is
associated with a second order EPD. We show that by introducing gain into
naturally lossy structures provides for the conditions whereby exceptional
points of non-Hermitian degeneracies can be manifested, such as in $\cal{PT}$-
symmetric structures. Furthermore, we also demonstrate that $\cal{PT}$-
symmetry, despite being the method often suggested for obtaining non-Hermitian
degeneracies at optical frequencies, is not a necessary condition and indeed we
show that EPD can be obtained with broken topological symmetry in uniform TLs.
Operating near such special degeneracy conditions leads to potential
performance enhancement in a variety of microwave and optical resonators, and
devices such as distributed oscillators, including lasers, amplifiers,
radiating arrays, pulse compressors, and Qswitching sensors.
