When molecules and clusters in the gas phase are excited by photons, electrons or heavier particles, a chain of events is kickstarted that may span twenty orders of magnitude in time. These processes involve, e.g., ultrafast attosecond electron dynamics and its coupling to nuclear motion and molecular rearrangements occurring on picosecond timescales, which may be followed by statistical redistribution of the internal energy that leads to an intricate competition between destructive and non-destructive relaxation pathways on timescales up to minutes and beyond. Moreover, how these processes are affected by the presence of a molecular environment is largely unknown. Advancing the fundamental understanding of gas-phase molecular dynamics therefore requires complementary state-of-the art experimental and theoretical methods designed to study specific processes and to monitor them on their associated timescales in unprecedented details.