Three and four flavor neutrino oscillations

We review some theoretical aspects of neutrino oscillations in the case where more than two neutrino flavors are involved. These include: approximate analytic solutions for 3-flavor (3f) oscillations in matter; matter effects in $\nu_\mu\leftrightarrow\nu_\tau$ oscillations; 3f effects in oscillations of solar, atmospheric, reactor, and supernova neutrinos and in accelerator long-baseline experiments; CP and T violation in neutrino oscillations in the vacuum and in matter; the problem of $U_{\mathrm{e3}}$; and 4f oscillations.