Only 15% of articles are currently being made Open Access (OA) through spontaneous self-archiving efforts by their authors. They average 25%-250% more citations in all 12 disciplines tested so far. Ninety-four percent of journals endorse immediate OA self-archiving. There is no evidence that self-archiving induces subscription cancellations. The “OA advantage” consists of: Early Advantage (early self-archiving produces both earlier and more citations), Usage Advantage (more downloads for OA articles, correlated with later citations), Competitive Advantage (relative citation advantage of OA over non-OA articles: disappears at 100% OA), Quality Advantage (OA advantage is higher, the higher the quality of the article) and Quality Bias (authors selectively self-archiving their higher quality articles – a non-causal component: disappears at 100% OA). We are currently comparing the OA advantage for mandated and spontaneous (self-selected) self-archiving. Deposit rates in Institutional Repositories (IRs) remain at 15% if unmandated, but climb toward 100% OA if mandated, confirming surveys that predicted 95% compliance. In the UK, 4 of the 8 research funding councils and the Wellcome Trust mandate self-archiving and it is being considered by the European Commission and the US federal FRPAA. There is no reason for universities to wait for the passage of the legislation. Five universities and two research institutions (including CERN) have already mandated it, with documented success. An Immediate-Deposit/Optional-Access Mandate covers all cases and moots all legal issues: metadata are immediately visible webwide and, where needed, access to the postprint can be set as Closed Access instead of OA throughout any embargo period. Software to support this approach (that allows the author to email individual copies of non-Open Access papers to individual requesters) has been created for both EPrints and DSpace repository platforms