  Turbulence is a widely observed state of fluid flows, characterized by
complex, nonlinear interactions between motions across a broad spectrum of
length and time scales. While turbulence is ubiquitous, from teacups to
planetary atmospheres, oceans and stars, its manifestations can vary
considerably between different physical systems. For instance,
three-dimensional (3D) turbulent flows display a forward energy cascade from
large to small scales, while in two-dimensional (2D) turbulence, energy
cascades from small to large scales. In a given physical system, a transition
between such disparate regimes of turbulence can occur when a control parameter
reaches a critical value. The behavior of flows close to such transition
points, which separate qualitatively distinct \textit{phases} of turbulence,
has been found to be unexpectedly rich. Here, we survey recent findings on such
transitions in highly anisotropic turbulent fluid flows, including turbulence
in thin layers and under the influence of rapid rotation. We also review recent
work on transitions induced by turbulent fluctuations, such as random reversals
and transitions between large-scale vortices and jets, among others. The
relevance of these results and their ramifications for future investigations
are discussed.
