A bronchopleural fistula (BPF) is fistula between a major, lobar, or segmental bronchus and the pleural space. An alveolopleural fistula (APF) is a fistula between the pulmonary parenchyma (alveoli) distal to a segmental bronchus and the pleural space. Several aetiological conditions are common to both BPF and APF. Nevertheless, the clinical implications, the trajectory and outcome of each is different. This chapter considers the differences between BPF and APF, and discusses the reasons behind a different clinical approach to each. It contextualises persistent air leaks and outlines various approaches to quantify and localise them. It also explores the different causes behind non-re-expansion of the lung and discusses the associated pathophysiology. It considers the different kinds of lung collapse, and discusses their aetiopathology. It also discusses the differences between lung entrapment and a trapped lung, and between drainage-dependent and drainage-independent air leaks.