The interaction of interplanetary shock waves with cometary atmospheres is described briefly in terms of 'shock-bow shock' and 'shock-tangential discontinuity' interactions. The tangential discontinuity is used to describe the boundary between the cometary and solar wind gases. Instability of the tangential discontinuity and its temporary breakup following impact by an interplanetary shock wave would, it is suggested, be followed by mixing between the solar wind and cometary plasma. It is also suggested that this physical process, part of which has already been studied extensively, is a principal factor responsible for cometary brightness variations following solar flare and/or stream-stream activity.