[en] A review is presented of reconnection in a space plasma of collisionless magnetic field lines with special regard to the earth's magnetosphere. The collisionless tearing mode is considered to be the main mechanism for the merging of magnetic field lines. It is shown that the difference between the nose and tail reconnection is related to the difference in the geometries of magnetic field lines. Non-linear analysis of the collisionless tearing mode provides an estimate of the reconnection rate of interplanetary and geomagnetic field lines. Magnetospheric tail instability during substorms is discussed in terms of the tearing-mode development. (author)