This chapter focuses on instability theory and nonlinear evolution of blocks and mature anomalies. The results of the studies that indicate that blocking and teleconnection patterns may be generated through the three-dimensional instability of atmospheric flows are presented. The instability properties of climatological flow for the Northern Hemisphere are examined. It is found that three-dimensional instability theory produces very low-frequency teleconnection patterns, such as the Pacific-North American and North Atlantic Oscillation patterns, as well as medium-frequency onset of blocking modes upstream of the mature patterns and higher frequency monopole cyclogenesis modes, which have maximum amplitudes in the observed storm tracks. It is observed that for the small-amplitude perturbation to the basic state, the fastest growing linear instability mode for the January 1978 basic state obtained from a three-dimensional instability calculation in a quasigeostrophic model is used. The shift in the maximum of the disturbance amplitude is qualitatively similar to that found in corresponding nonlinear experiments of disturbances growing on zonally averaged basic flows.