The spin-wave spectra in the antiferromagnetic, ferrimagnetic, and ferromagnetic phases of Fe${\mathrm{Cl}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 2${\mathrm{H}}_{2}$O have been examined at 2 and 6 \ifmmode^\circ\else\textdegree\fi{}K. These spectra are well described by a spin-wave calculation; it is shown that ${g}_{\ensuremath{\parallel}}=2.23\ifmmode\pm\else\textpm\fi{}0.02$, $S=2$, and that the exchange interactions are isotropic. The (large) longitudinal anisotropy is included in the Hamiltonian as a single-ion anisotropy ($D=+9.58\ifmmode\pm\else\textpm\fi{}0.05$ ${\mathrm{cm}}^{\ensuremath{-}1}$), which is shown to make an anomolously large contribution to the spin-wave energies. These results are discussed and interpreted from the point of view of crystal-field theory. In all three metamagnetic phases, the magnetic resonance modes are observed to interact with a field-independent excitation with energy 31.5 ${\mathrm{cm}}^{\ensuremath{-}1}$, which is presumably an optical phonon. The measured value of the metamagnetic transition field ${\mathrm{H}}_{\mathrm{c}2}=45.0\ifmmode\pm\else\textpm\fi{}0.5$ kOe compares quite well with Narath's value of 45.6 kOe, but our value of ${\mathrm{H}}_{c1}=35.0\ifmmode\pm\else\textpm\fi{}0.5$ kOe is in poor agreement with Narath's 39.2 kOe. Near ${\mathrm{H}}_{c1}$ (the antiferromagnetic-to-ferrimagnetic transition field), the far-infrared spectrum appears to indicate that both antiferromagnetic and ferrimagnetic coexist over a certain range of field. The temperature dependence and hysteresis of these domains are also described and compared with Tinkham's microscopic description of these transitions.