  Rare events are processes that occur upon the emergence of unlikely
fluctuations. Unlike what their name suggests, rare events are fairly
ubiquitous in nature, as the occurrence of many structural transformations in
biology and material sciences is predicated upon crossing large free energy
barriers. Probing the kinetics and uncovering the molecular mechanisms of
possible barrier crossings in a system is critical to predicting and
controlling its structural and functional properties. Due to their activated
nature, however, rare events are exceptionally difficult to study using
conventional experimental and computational techniques. In recent decades, a
wide variety of specialized computational techniques-- known as advanced
sampling techniques-- have been developed to systematically capture improbable
fluctuations relevant to rare events. In this perspective, we focus on a
technique called forward flux sampling (Allen~\emph{et al.,}~\emph{J. Chem.
Phys.}, {\bf 124}: 024102, 2006), and overview its recent methodological
variants and extensions. We also provide a detailed overview of its application
to study a wide variety of rare events, and map out potential avenues for
further explorations.
