  We show that a common feature of temperature-dependent microwave absorption
is the presence of absorption peaks. $ac$ loss peaks can arise when the
internal $T$-dependent magnetic relaxation time crosses the measurement
frequency. These features are observed in the \QTR{em}{insulating}
($Sr_{x}Ca_{14-x}Cu_{24}O_{41}$, $La_{5/3}Sr_{1/3}NiO_{4}$ and
$YBa_{2}Cu_{3}O_{6.0}$), \QTR{em}{pseudo-gap} ($T>T_{c}$ in underdoped
$YBa_{2}Cu_{3}O_{7-\delta}$, $Hg:1223$ and $Hg:1201$) and
\QTR{em}{superconducting} ($T<T_{c}$) states of the oxides. The commonality of
these features suggests a magnetic (spin) mechanism, rather than a
quasiparticle origin, for the so-called ``conductivity'' peaks observed in the
cuprate superconductors.
