  Bell inequalities are intended to show that local realist theories cannot
describe the world. A local realist theory is one where physical properties are
defined prior to and independent of measurement, and no physical influence can
propagate faster than the speed of light. Quantum-mechanical predictions for
certain experiments violate the Bell inequality while a local realist theory
cannot, and this shows that a local realist theory cannot give those
quantum-mechanical predictions. However, because of unexpected circumstances or
"loopholes" in available experiment tests, local realist theories can reproduce
the data from these experiments. This paper reviews such loopholes, what effect
they have on Bell inequality tests, and how to avoid them in experiment.
Avoiding all these simultaneously in one experiment, usually called a
"loophole-free" or "definitive" Bell test, remains an open task, but is very
important for technological tasks such as device-independent security of
quantum cryptography, and ultimately for our understanding of the world.
