  The nonlinear memory effect is a slowly-growing, non-oscillatory contribution
to the gravitational-wave amplitude. It originates from gravitational waves
that are sourced by the previously emitted waves. In an ideal
gravitational-wave interferometer a gravitational-wave with memory causes a
permanent displacement of the test masses that persists after the wave has
passed. Surprisingly, the nonlinear memory affects the signal amplitude
starting at leading (Newtonian-quadrupole) order. Despite this fact, the
nonlinear memory is not easily extracted from current numerical relativity
simulations. After reviewing the linear and nonlinear memory I summarize some
recent work, including: (1) computations of the memory contribution to the
inspiral waveform amplitude (thus completing the waveform to third
post-Newtonian order); (2) the first calculations of the nonlinear memory that
include all phases of binary black hole coalescence (inspiral, merger,
ringdown); and (3) realistic estimates of the detectability of the memory with
LISA.
