We study kaonic hydrogen, the bound K - p state A K p . Within a quantum field theoretic and relativistic covariant approach we derive the energy level displacement of the ground state of kaonic hydrogen in terms of the amplitude of K - p scattering for arbitrary relative momenta. The amplitude of low-energy K - p scattering near threshold is defined by the contributions of three resonances $\Lambda(1405)$ , $\Lambda(1800)$ and $\Sigma^0(1750)$ and a smooth elastic background. The amplitudes of inelastic channels of low-energy K - p scattering fit experimental data on the near-threshold behaviour of the cross-sections and the experimental data by the DEAR Collaboration. We use the soft-pion technique (leading order in Chiral Perturbation Theory) for the calculation of the partial width of the radiative decay of pionic hydrogen $A_{\pi p} \to n + \gamma$ and the Panofsky ratio. The theoretical prediction for the Panofsky ratio agrees well with experimental data. We apply the soft-kaon technique (leading order in Chiral Perturbation Theory) to the calculation of the partial widths of radiative decays of kaonic hydrogen $A_{Kp} \to \Lambda^0 + \gamma$ and $A_{K p} \to \Sigma^0 + \gamma$ . We show that the contribution of these decays to the width of the energy level of the ground state of kaonic hydrogen is less than 1 $\%$ .