Perfect Constant-Weight Codes

In his pioneering work from 1973, Delsarte conjectured that there are no nontrivial perfect codes in the Johnson scheme. Many attempts were made, during the years which followed, to prove Delsarte's conjecture, but only partial results have been obtained. We survey all these attempts, and prove some new results having the same flavor. We also present a new method, taking a different approach, which we hope can lead to the settling of this conjecture. We show how this new method rules out sets of parameters as well as specific given parameters.