Blazars are a special subclass of active galaxy nucleis (AGNs). Radiation from blazars is thought to originate in a relativistic jet oriented at a small angle with respect to the line of sight. They usually show rapid variability, high polarization and strong non-thermal emission. Spectral energy distributions (SEDs) of blazars are characterized by two broad components: a low-energy component from radio to X-ray frequency, and a high-energy component from X-ray to $\gamma$-ray. It is generally accepted that the low-energy component of blazars SEDs is produced by synchrotron radiation from relativistic electrons in the jet. However, the origin of high-energy component is still a matter of debate. There are two classes of models that are used to explain the high-energy emission of blazars: the leptonic model and the hadronic model. In the leptonic model, the high-energy emission is produced by inverse Compton scattering of electrons on background photons. In the hadronic model, the high-energy emission originates from proton synchrotron or photon-hadronic interactions. We study the origin of high-energy emission from blazars based on leptonic model and the hadronic model. The main results are as following: 1. We study the location of the GeV emission region of 21 flat spectrum radio quasars (FSRQs) with quasi-simultaneous spectral energy distributions. We propose a method to constrain the location of the GeV emission region based on the spectral shapes. If the $\gamma$-ray emission region is located inside the BLR, the IC scattering could occur at the Klein-Nishina (KN) regime and the $\gamma$-ray spectrum should be steeper than the optical-infrared spectrum. If the $\gamma$-ray emission region is located far beyond the the broad-line region (BLR), the IC scattering could take place at the Thomson regime and the $\gamma$-ray spectrum should have the same spectral index as the optical-infrared spectrum. In order to test our scenario, We reproduced the simultaneous SEDs of 21 FSRQs using one-zone leptonic model with the synchrotron-self Compton (SSC) and external Compton (EC) processes. We suggest that the X-ray emission is produced by SSC process, the GeV emission comes from the external Compton (EC) process, in which the EC emission may originate from the inverse Compton (IC) scattering of photons from BLR and accretion disc or dust torus by the same electron population. We infer from the spectral shapes and SED modeling that the location of the GeV emission region is inside the BLR for 5 FSRQs and beyond the BLR for 16 FSRQs. Our results show that the ratio of the magnetic field and electron energy density is close to equipartition condition for 21 FSRQs. 2. We investigate the X-ray and $\gamma$-ray flares of Mrk 421 on 2008 June 6-15 using the SSC model with electron acceleration, in which an evident correlation between the X-ray and $\gamma$-ray bands appears, while no significant correlation between the optical and X-ray band is observed. We argue that the emission from Mrk 421 may originate from two different components. One is the steady component from the outer region that is mainly attributed to the optical radiation, in which the electrons are accelerated by first-order Fermi acceleration mechanism. We use a steady electron spectrum to produce the SSC emission from the steady component. The other is the variable component from the inner region, in which the electrons are accelerated by the stochastic acceleration process. We use the time-dependent SSC model to produce the emission from the variable component. We suggest that the flares are due to the hardening of the electron spectrum under the process of the stochastic acceleration, which leads to the hardening of the observed spectrum in the X-ray and $\gamma$-ray bands. Furthermore, we find that the energy densities of electrons and magnetic fields are near equipartition in both jet regions. 3. The very hard $\gamma$-ray spectrum from distant blazars chall
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