On the nature of the dark mass in the centre of the Milky Way

We discuss constraints on the properties and nature of the dark mass concentration at the core of the Milky Way. We present 0.15-arcsec astrometric K-band maps in five epochs beween 1992 and 1996. From these we derive imposed stellar proper motions within 3 arcsec of the compact radio source Sgr A * whose infrared counterpart may have been detected, for the first time, in a deep image in 1996 June.