  Weakly Interacting Massive Particles (WIMPs) have long reigned as one of the
leading classes of dark matter candidates. The observed dark matter abundance
can be naturally obtained by freezeout of weak-scale dark matter annihilations
in the early universe. This "thermal WIMP" scenario makes direct predictions
for the total annihilation cross section that can be tested in present-day
experiments. While the dark matter mass constraint can be as high as
$m_\chi\gtrsim100$ GeV for particular annihilation channels, the constraint on
the total cross section has not been determined. We construct the first
model-independent limit on the WIMP total annihilation cross section, showing
that allowed combinations of the annihilation-channel branching ratios
considerably weaken the sensitivity. For thermal WIMPs with s-wave
$2\rightarrow2$ annihilation to visible final states, we find the dark matter
mass is only known to be $m_\chi\gtrsim20$ GeV. This is the strongest largely
model-independent lower limit on the mass of thermal-relic WIMPs, together with
the upper limit on the mass from the unitarity bound ($m_\chi\lesssim 100$
TeV), it defines what we call the "WIMP window". To probe the remaining mass
range, we outline ways forward.
