Potter Stewart

“ The Court confines itself to deciding the policy considerations that, in our federal system, must prevail when federal courts are asked to interfere with pending state prosecutions. Within this area, we hold that a federal court must not, save in exceptional and extremely limited circumstances, intervene by way of either injunction or declaration in an existing state criminal prosecution. [4] Such circumstances exist only when there is a threat of irreparable injury "both great and immediate." ”

“ This opinion applies also to No. 7, Samuels et al. v. Mackell et al., and No. 9, Fernandez v. Mackell et al., post, p. 66; No. 41, Dyson et al. v. Stein, post, p. 200; and No. 83, Byrne et al. v. Karalexis et al., post, p. ”