The research question of my PhD is in a way a simple one: what can observations of the He I D3 line teach us about the solar chromosphere? This optical spectral line at 5876 Å is formed in the upper chromosphere, and is sensitive to the local magnetic field and to heating of the transition region and corona. In order to explain why, I will discuss the unconventional formation mechanism of He I D3. This line was a commonly used diagnostic for the solar chromosphere in the 70’s for flares and later also for prominences. However, the spatial resolution of the data available is very poor compared to what we can achieve today at for example the SST. During my PhD, I have focused on high-resolution observations of He I D3 with different instruments at the SST, and in co-observation with space-borne instruments. These observations have provided new insights in the reconnection events that we have studied. Our He I D3 observations have set constraints on the temperature of Ellerman Bombs (EBs, i.e. tiny reconnection events in the upper photosphere) – which is relevant in the context of chromospheric heating. Currently, 3D radiative transfer calculations are undertaken with the goal of understanding helium line formation in small-scale reconnection events. In the context of flares, we measured strong downflows in the chromosphere via He I D3, revealing shocks and dynamics during a flare. Spectro-polarimetry was used to measure the magnetic field during a flare and to propose its magnetic topology. In summary, the He I D3 line is an excellent probe for high-energy processes in the chromosphere and many possible applications are yet to be exploited.