Recently, air lasing has attracted significant attention owing to its promising applications in atmospheric sensing and environmental monitoring. Air lasing usually refers to strong-laser-induced population inversion of atmospheric constituents, resulting in no-cavity light amplification over a remote distance. It has been revealed that with the excitation of intense laser pulses, molecules of the two primary atmospheric constituents, i.e., nitrogen and oxygen, can exhibit lasing behaviors. The gain media can be atomic oxygen and nitrogen, as well as neutral nitrogen molecules and nitrogen molecular ions. Herein, we present the phenomena, underlying mechanisms, and potential applications of air lasing with a focus on the gain media of neutral and single-ionized nitrogen molecules. Moreover, the influence of the laser polarization state on the N2+ lasing is discussed in more detail.