To cope with variations, disturbances, and unexpected events in complex socio-technical systems people are required to continuously adapt to the changing environment, sometimes in novel and innovative ways. This thesis investigates adaptive performance in complex work settings across domains, with a focus on examining what enables and disables successful adaptations, and how contextual factors shape performance. Examples of adaptive performance studies include a crisis command team dealing with the loss of key personnel, a crew coping with unreliable system feedback in the cockpit, and a nursing team managing an overload of patients. The two main contributions of this thesis is the analysis of cases of people coping with variations and disturbances, and the development of conceptual models to report findings, structure cases, and make sense of sharp-end adaptations in complex work settings. The findings emphasise that adaptive performance outside procedures and textbook scenarios at the sharp end is a critical ability to cope with variation and unexpected events. However, the results also show that adaptations may come at the cost of new vulnerabilities and system brittleness. Analysing adaptive performance in everyday events informs safety management by making visible limitations and possibilities of system design, organisational structures, procedures, and training.