The aim of this paper is to develop an 'external' Kurosh-Amitsur radical theory of semirings and, using this approach, to obtain some fundamental results regarding two Jacobson type of radicals -the Jacobson-Bourne, J-, radical and a very natural its variation, J sradical -of hemirings, as well as the Brown-McCoy, R BM -, radical of hemirings. Among the new central results of the paper, we single out the following ones: Theorems unifying two, internal and external, approches to the Kurosh-Amitzur radical theory of hemirings; A characterization of J-semisimple hemirings; A description of J-semisimple congruence-simple hemirings; A characterization of finite additively-idempotent J s -semisimple hemirings; Complete discriptions of R BM -semisimple commutative and latticeordered hemirings; Semiring versions of the well-known classical ring results-Nakayama's and Hopkins Lemmas and Jacobson-Chevalley Density Theorem; Establishing the fundamental relationship between the radicals J, J s , and R BM of hemirings R and matrix hemirings M n (R); Establishing the matric-extensibleness (see, e.g., [4, Section 4.9]) of the radical classes of the Jacobson, Brown-McCoy, and J s -, radicals of hemirings; Showing that the J-semisimplicity, J s -semisimplicity, and R BM -semisimplicity of semirings are Morita invariant properties.