  At even-denominator Landau level filling fractions, such as $\nu=1/2$, the
ground state, in most cases, has no energy gap, and there is no quantized
plateau in the Hall conductance. Nevertheless, the states exhibit non-trivial
low-energy phenomena. Open questions concerning the proper description of these
systems have attracted renewed attention during the last few years. Issues at
$\nu=1/2$ include consequences of particle-hole symmetry, which should be
present for a spin-aligned system in the limit where one can neglect mixing
between Landau levels. Other issues concern questions of anisotropy and
geometry, properties at non-zero temperature, and effects of relatively strong
disorder. In cases where one does find a gapped even-denominator quantized Hall
state, such as $\nu=5/2$ in GaAs structures, major questions have arisen about
the nature of the quantum state, which will be discussed briefly in this
chapter. The chapter will also discuss phenomena that can occur in a
two-component system near half filling, i.e., when the total filling factor
$\nu_{\rm{tot}} $ is close to 1.
