Solar wind/IMF parameters and their variations influence the state and dynamics of the magnetosphere in several different ways, and the plasma sheet plays its own active role to form the magnetotail's response to external driving. This field is still quite controversial; key unsolved issues are those which probably involve the nonsteady, nonequilibrium and nonlinear character of the system. This paper discusses an interesting development of concepts concerning (1) plasma sheet convection, (2) mechanisms which initiate the substorm onset, and (3) variability of tail dynamics. A remarkable example is a recognition of Bursty Bulk Flows as a basic way for the plasma sheet to sustain the convective transport of plasma, energy and magnetic flux via transient mesoscale dynamic structures. As concerns the substorm onset, the sharp change from the magnetic reconnection (NENL) model to processes in the dipolar-like near tail is now moving into a synthetic stage. Here non-linear models of different coupled instabilities are actively explored to find a way to excite the tearing mode starting from a singular thin current sheet configuration which seems to form in the near tail prior to the breakup. Variable responses to external drivers and the variability of substorms compel a recognition of the magnetotail as a very complicated nonlinear open system which includes a hierarchy of coupled processes of different scales. A unique fleet of magnetospheric spacecraft, simultaneously probing different domains and supported by extensive ground observations, global imaging and solar wind monitoring, now provides a real chance to understand the magnetotail as a global dynamic system. (C) 1999 Elsevier Science Ltd. All rights reserved.