  Star clusters are subject to density irregularities in their host galaxy,
such as giant molecular clouds (GMCs), the galactic disc and spiral arms, which
are largely ignored in present day (N-body) simulations of cluster evolution.
Time dependent external potentials give rise to tidal forces that accelerate
stars leading to an expansion and more rapid dissolution of the cluster. I
explain the basic principles of this tidal heating in the impulse approximation
and show how related disruption time-scales depend on properties of the
cluster.
