Iron complexes with tetradentate N 4-donor aminopyridine ligands efficiently catalyze the enantioselective epoxidation of olefins with hydrogen peroxide in the presence of carboxylic acids (RCOOH), used as catalytic additives. In the catalyst systems (L)Fe/H2O2/RCOOH, two types of iron-oxo intermediates have been observed by EPR spectroscopy: the low-spin intermediates with large g-factor anisotropy (g 1 ~ 2.7, g 2 ~ 2.4, g 3 ~ 1.7) and the low-spin intermediates with small g-factor anisotropy (g 1 = 2.07, g 2 = 2.01, g 3 = 1.96), both capable of directly reacting with olefinic substrates. Intermediates of the latter type demonstrate superior epoxidation enantioselectivity. The catalytic and EPR spectroscopic studies of the aforementioned catalyst systems have revealed the primary factors, controlling the electronic structure and reactivity of the iron-oxo intermediates: (i) the electron-donating (or withdrawing) properties of substituents at the pyridine rings; and (ii) the structure of carboxylic acid additive. The combined EPR spectroscopic, catalytic, kinetic, and isotopic labeling data obtained for the catalyst systems with different oxidants provide evidence that the same actual epoxidizing species - oxoiron complexes [(L)FeV-O(OC(O)R)]2+ operate in the catalyst systems (L)Fe/H2O2/RCOOH and (L)Fe/R1OOH/RCOOH (R1 = H, t-butyl or cumyl). On the contrary, in the systems with peroxycarboxylic acids as oxidants, i.e. (L)Fe/R2C(O)OOH (R2 = CH3 or 3-Cl-C6H4), in the presence or in the absence of carboxylic acid, the epoxidation is predominantly conducted by the iron(III)-acylperoxo intermediates [(L)FeIII(OOC(O)R2)]2+. The oxoiron intermediates (both those with large and small g-factor anisotropy) have also been identified as the active species, responsible for the C-H oxidation of cyclohexane and adamantane in the catalyst systems (L)Fe/H2O2/RCOOH and (L)Fe/CH3CO3H/RCOOH (RCOOH = acetic or 2-ethylhexanoic acid). Curiously, in the catalyst systems 4/m-CPBA/RCOOH and 4*/m-CPBA/RCOOH, using bipyrrolidine-based iron complexes as catalysts and m-CPBA as the terminal oxidant, it is apparently the iron(III)-m-chlorobenzoylperoxo intermediates that predominantly contribute to the C-H oxidations mediated by these systems.