The charge-density distribution in materials dictates their chemical bonding, electronic transport, and optical and mechanical properties. Indirectly measuring the charge density of bulk materials is possible through X-ray or electron-diffraction techniques by fitting their structure factors1-3, assuming that the sample is perfectly homogeneous within the beam illuminated area. The recent development of scanning tunnelling microscopy and atomic force microscopy enables us to see chemical bonds, but only on surfaces4-6. It therefore remains a challenge to resolve charge density in nanostructures and functional materials with imperfect crystalline structures, for example those with defects, interfaces or boundaries where new physics emerges. Here we describe the development of a real-space imaging technique that can directly map the local charge density of crystalline materials, using scanning transmission electron microscopy alongside an angle-resolved pixelated fast electron detector. Using this technique, we imaged the interfacial charge distribution and ferroelectric polarization in a SrTiO3/BiFeO3 heterojunction and discovered charge accumulation at the interface that was induced by the penetration of the polarization field of BiFeO3, which is validated through side-by-side comparison with density functional theory calculations. The charge-density imaging method established in this work advances electron microscopy from detecting atoms to imaging electrons, paving a new route towards study local bonding in crystalline solids.