  We briefly review the studies on the top quark FCNC processes at a
next-generation linear collider. Such processes, including various FCNC top
quark rare decays and top-charm associated productions, are extremely
suppressed in the standard model (SM) but could be significantly enhanced in
some extensions. We compared the predictions from different typical new physics
models: the SM, the minimal supersymmetric model (MSSM), the general
two-higgs-doublet model (2HDM) and the topcolor-assisted technicolor (TC2)
model. Our conclusion is: (1) While all the new physics models can enhance the
rates by several orders relative to the SM predictions, the TC2 model predicts
much larger rates than other models; (2) The optimal channel for probing the
top quark FCNC is the top-charm associated production in \gamma-\gamma
collision, which occurs at a much higher rate than e^+ e^- or e^- \gamma
collision and can reach the detectable level for a large part of the parameter
space.
