The paper attempts to explain the causes of phasmatid outbreaks in a theory of outbreak release which is based on direct observations of the phasmatid Didymuria violescens (Leach) and a subjective analysis of several outstanding features associated with the history, distribution, and development of phasmatid outbreaks in the highlands of south-eastern Australia. The outbreaks of D. violescens are largely confined to mountain forests in southern New South Wales and Victoria where a 2-yr generation period predominates and a 2-yr cycling of population prevails with peaks and troughs of hatching, and hence "peak" and "off" years of phasmatid abundance, in alternate years. Birds greatly limit the survival rate of D. violescens during off-years but are ineffective during peak years because of their limited total response to increase in numbers of the prey. The theory implies that D. violescens has two systems of control: one at low density when increase is limited largely by egg parasites or birds or both; and another at high density when increase is ultimately prevented by intraspecific competition for food. The theory postulates that very cool summer weather conditions may occasionally disrupt the low-density system of control and thereby initiate outbreak release either directly in some situations or indirectly in others.