The aim of this study was to develop a classic grounded theory of patients, relatives and nurses in palliative cancer care. Data from three earlier studies conducted in palliative care were analyzed. “Deciphering unwritten rules” emerged as the pattern of behavior through which patients, relatives and nurses are dealing with the uncertainty of how to act and behave in palliative cancer care. Deciphering means finding out what the rules mean and trying to interpret them and this can be done consciously or unnoticed. Deciphering unwritten rules involves the strategies figuring out, deliberating, maneuvering and evaluating. This theory demonstrates the complexities of palliative care and the importance of knowledge, counseling and resources for all involved.