  We analyze {\it K2} light curves for 794 low-mass ($1 > M_* > 0.1$
$M_{\odot}$) members of the $\approx$650-Myr-old open cluster Praesepe, and
measure rotation periods ($P_{rot}$) for 677 of these stars. We find that half
of the rapidly rotating $>$0.3 $M_{\odot}$ stars are confirmed or candidate
binary systems. The remaining $>0.3$ $M_{\odot}$ fast rotators have not been
searched for companions, and are therefore not confirmed single stars. We found
previously that nearly all rapidly rotating $>$0.3 $M_{\odot}$ stars in the
Hyades are binaries, but we require deeper binary searches in Praesepe to
confirm whether binaries in these two co-eval clusters have different $P_{rot}$
distributions. We also compare the observed $P_{rot}$ distribution in Praesepe
to that predicted by models of angular-momentum evolution. We do not observe
the clear bimodal $P_{rot}$ distribution predicted by Brown (2014) for $>$0.5
$M_{\odot}$ stars at the age of Praesepe, but 0.25$-$0.5 $M_{\odot}$ stars do
show stronger bimodality. In addition, we find that $>$60\% of early M dwarfs
in Praesepe rotate more slowly than predicted at 650 Myr by Matt et al. (2015),
which suggests an increase in braking efficiency for these stars relative to
solar-type stars and fully convective stars. The incompleteness of surveys for
binaries in open clusters likely impacts our comparison with these models,
since the models only attempt to describe the evolution of isolated single
stars.
