Skyrmions are nanoscale spin textures that are viewed as promising candidates as information carriers in future spintronic devices". Skyrmions have been observed using neutron scatterne'S and microscopy techniques6-11. Real-space imaging using electrons is a straightforward way to interpret spin configurations by detecting the phase shifts due to electromagnetic fields. Here, we report the first observation by electron holography of the magnetic flux and the three-dimensional spin configuration of a skyrmion lattice in FeasCoosSi thin samples. The magnetic flux inside and outside a skyrmion was directly visualized and the handedness of the magnetic flux flow was found to be dependent on the direction of the applied magnetic field. The electron phase shifts q, in the helical and skyrmion phases were determined using samples with a stepped thickness t (from SS nm to 510 nm), revealing a linear relationship (tp = 0.001730. The phase measurements were used to estimate the three-dimensional structures of both the helical and skyrmion phases, demonstrating that electron holography is a useful tool for studying complex magnetic structures and for three-dimensional, real-space mapping of magnetic fields12.