The phase specific model for the prevention and control of sexually transmitted infections (STI) offers new insights into the strategic planning of programmes. The model illustrates the importance of modifying the focus of prevention and control activities to different subpopulations as the epidemic evolves over time. However, the practical application of phase specific approaches will depend on an understanding of the variability and determinants in the trajectory by which STI epidemics progress through epidemic phases. This paper draws on empirical observations from diverse populations to explore the influence of sexual behaviour patterns in populations, the biological characteristics of STI pathogens, and the population–pathogen interactions in relation to epidemic trajectories. In addition, various approaches to the determination of epidemic phase are presented.