Variable red giants populate several sequences in the period-luminosity plane. Most of these sequences are usually attributed to
different modes of radial pulsation in asymptotic giant branch (AGB) stars, but new models are providing alternative
interpretations. Other sequences represent less common types of variability, such as ellipsoidal binaries and long secondary
periods. While these types of variability are slowly being elucidated, many unanswered questions remain. Still other AGB stars
show forms of variability that are non-periodic, which may be even less understood.
I will review AGB variability, touching on both the known and unknown forms of variability, recent research, and presenting the
major questions yet unsolved.
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