Section 1.2 of this introductory chapter discusses what patient safety and patient harm mean when applied to primary and ambulatory care. This reflection is necessary because safety and harm are traditionally approached in the context of acute care. Owing to the fundamental differences between acute services and primary/ambulatory care, the definitions and concepts need to be refined for the purpose here. Chapter 2 examines the occurrence, frequency and severity of safety lapses in primary and ambulatory care. It explores different avenues for capturing patient harm in primary and ambulatory care settings, shedding light on shortcomings of methodologies, knowledge gaps and the main causes of harm. At the margin, the cost of preventing every last case of harm would approach, and eventually exceed, the costs of failure, principally because - in a system with finite resources - beneficial activity would need to be diverted, or stopped altogether, in order to remove all risk (see section 1.2).