Care workers are knowledge workers in the sense that their knowledge and skills substantially influence the quality of care. However, the types of knowledge needed by care workers and the ways they expand their knowledge are different from those suggested for typical knowledge workers, who basically acquire and update theoretical knowledge through formal education and training. Using a survey data of Japanese home care workers, it is empirically demonstrated that the quality of care depends on the contexts shared between care workers and their clients, and therefore for care workers to provide effective care, understanding and constructing the appropriate contexts for care in relation with various clients is necessary and primary. It was found that knowledge needed in understanding and constructing the appropriate contexts for care is leant more effectively through collective work, dialogue and reflection with fellow workers than through formal education and training.