We have analyzed collusion in multi-unit auctions under VCG payments, and shown that such a domain is extremely valunerable to collusion. The colluders can easily find the optimal manipulation, and split the gains from the collu- sion in a stable and fair manner. Our results indicate the colluders can not only find a stable way to distribute the gains, they can even use a distribution that fairly allocates each colluder a share of the utility reflecting her contribu- tion to the colluders’ utility. Due to Theorem 5, in many domains the colluders can even use a simple polynomial al- gorithm to compute this utility distribution. These results indicate that many factors operate in favour of the collud- ers in this domain, so significant counter-measures must be used to stop collusion. We now present several questions that remain open for future research.