In this talk, I will discuss how neutrinos lose coherence during propagation and how additional mixing of states (leading to new coherence) can occur. The former will focus on precision measurements of baseline neutrino oscillation and what information we can learn from the oscillation pattern due to decoherence; the later aims at high energy astrophysical neutrinos, where a mixing between opposite helicity states can be induced by the magnetic field required for particle acceleration.