Tremendous numbers of papers have been published recently regarding non-chlorine or less chlorine bleaching sequences. Although the toxicities of chlorinated organic compounds in pulp bleach effluent have been discussed tens of years, definitely the US EPA report on chlorinated dioxins has pushed us toward non-chlorine or less chlorine bleaching processes. Oxygen pre-bleaching is now becoming a standard first stage bleaching process of kraft pulp, although US is behind. Substitution by chlorine dioxide for elementary chlorine to different extent is also being widely used in the mills. Some new trials such as pretreatment oxygen delignification or improved ozone bleaching are also proposed. The objectives of this paper is firstly to have better understanding on the structures and reactivities of the residual lignin in kraft pulp and secondly to find the key for better non-chlorine bleaching processes.