F-theoretic constructions can alternatively be understood as consequences of certain N = 2 Seiberg-Witten theories via type IIB r D3-branes probing the quantum corrected orientifold backgrounds. We present four models that come out from such consideration.In Model 1, the seven-branes wrap the flat R 4 directions, leading to the well known Sp(2r) theories. We study singularity structure of moduli space of Seiberg-Witten curve, such as maximal Argyres-Douglas loci, in order to construct 1-1 map between moduli spaces.In Model 2, the seven-branes are wrapped on Taub-NUT and multi Taub-NUT spaces instead of R 4 . These configurations may succinctly explain many of the recently proposed Gaiotto-type constructions including possible extensions to non-conformal models with interesting cascading behaviors. In this model the UV limit would be described by the probe D3-branes decomposed into D5-D5 pairs wrapped on vanishing 2-cycles of multi Taub-NUT space, while the IR would remain a 4d theory. For certain arrangements of the seven-branes and in a specific delocalization limit, this model may be dualized to the brane networks recently studied by Benini, Benvenuti and Tachikawa. On the other hand, the Gaiotto dualities in Model 2 are explained by three simultaneous effects: chiral anomaly cancellations, anti-GSO projections and brane transmutations.Model 3 is described by seven-branes wrapped on a K3 manifold and D3 and D3 probes whose number may differ at most by 24. These constructions could lead to new N = 2 models with possible dualities to both type IIB and heterotic theories on non-Kähler manifolds, and involve interesting interplays between abelian instantons and fluxes. In the limit where the number of probes becomes very large, the physics is captured by M(atrix) theory on K3 × K3 manifold with G-fluxes.Finally, Model 4 is described by k D3-branes probing intersecting seven-brane backgrounds with Sp(2k) × Sp(2k) gauge group. These constructions could produce new N = 1 heterotic dual given by k small instantons wrapping 2-cycles of a non-Kähler K3 manifold that is no longer conformally Calabi-Yau. We discuss possible constraints on these models coming from global charge and anomaly cancellations in F-theory and study the implications of these constraints on the global and local symmetries of the underlying gauge theories. Contents 65 4.4 N = 1, 2 examples and geometric dualities 68 5 Model 4: Multiple D3's probing intersecting seven-brane backgrounds 71 5.1 The heterotic dual with fluxes and five-branes 71 5.2 Type IIB analysis at the orientifold point 77 6 Discussion and conclusions 82 A How to obtain Seiberg-Witten curve and 1-form for pure Sp(2r) theories 88 -i -B Speculative viewpoints of Model 1 related to one-on-one mapping 90 B.1 A naive 1-1 mapping between moduli spaces of brane and curve 90 B.2 A fantasy on how to save the naive 1-1 mapping between moduli spaces of Seiberg-Witten curve and D3-brane picture for pure Sp(2r) theory 92 C Higgsing and Dualities for Sp(2r) 95 C.1 Higgsing of Sp(2r) 96 D Modular p...