Spectral series of the kilonova AT2017gfo showing the evolution over the first 10 days post-merger. This spectral compilation of data includes the early spectra taken with the Magellan telescopes (Shappee et al. 2017), ANU 2.3 m telescope (Andreoni et al. 2017) and SALT (Buckley et al. 2018). From 1.4 days post-merger, the X-shooter spectrograph at the European Southern Observatory’s VLT detailed the UV to NIR structure with a daily cadence (Pian et al. 2017; Smartt et al. 2017). To constrain the rapid evolution at early times, we also include the 1.5 day Gemini FLAMINGOS-2 and SOAR spectra (Nicholl et al. 2017; Chornock et al. 2017). Towards intermediate and late-times, HST (Tanvir et al. 2017) infrared spectroscopy additionally constrains the spectral structure in the telluric region around 1400 nm. The spectral continuum is well described as a blackbody in all photospheric epochs and even into nebular-phase epochs with a blackbody temperature evolution that is detailed in Sect. 4. In the zoom-in sub-panels, individual spectral lines are highlighted at different epochs, including (1) the 760 nm (2) 1.0 μm, and (3) 1.4 μm P Cygni features, and (4) the NIR line-emission features.