The authors describe how the definition of work-load in nursing is susceptible to many variables such as the number of patients cared for and their specific needs, the mix of skills required and the activities performed by nurses not strictly pertinent to care. These features make it difficult to create a global method for allocation of nursing resources . The authors have reviewed the literature from 2000 to 2009 in an attempt to identify the relationships between the concepts at the basis of the definition of work-load in nursing. The concepts of complexity and intensity of care are analyzed together with the various systems used for allocation of resources in different countries. Their results show that work-load is an important element of nursing organization and can influence the outcome of care.