Abstract. Nonlinear effects on the free evolution of three-dimensional disturbances are discussed, these disturbances having a spot-like character sufficiently far downstream of the initial disturbance. The inviscid initial-value formulation taken involving the three-dimensional unsteady Euler equations offers hope of considerable analytical progress on the nonlinear side, as well as being suggested by some of the experimental evidence on turbulent spots and by engineering modelling and previous related theory. The large-time large-distance behaviour is associated with the two major length scales, proportional to (time) 1/2 and to (time), in the evolving spot; within the former scale the Euler flow exhibits a three-dimensional triple-deck-fike structure; within the latter scale, in contrast, there are additional time-independent scales in operation. As the typical disturbance amplitude increases, nonlinear effects first enter the reckoning in edge layers near the spot's wing-tips. The nonlinearity is mostly due to interplay between the fluctuations present and the three-dimensional mean-flow correction which varies relatively slowly. The resulting amplitude interaction points to a subsequent flooding of nonlinear effects into the middle of the spot. There it is suggested that the fluctuation/mean-flow interaction becomes strongly nonlinear, substantially altering the mean properties in particular. A new global viscous-inviscid interaction between the short and long scales present, involving Reynolds stresses, is also identified. The additional significance of viscous sublayer bursts is also noted, along with comments on links with experiments and direct numerical simulations, on channel flows and jets, and on further research.