The Information Systems literature observes that, while there are ostensibly benefits to sharing of data and information, barriers to organisational data sharing appear significant. Managers may be understandably concerned that the sharing activity is adversely affecting their own organisations. This paper develops a model of data and information sharing based on the traditional system model, and proposes a theory of the sharing activity in organisations. The paper theorises that employees may engage in or oppose sharing based on the assessment of perceived benefits accruing to themselves from the activity. In particular, the paper highlights the contention that data and information sharing are likely to decrease as organisations grow in size, and also offers an explanation for why the sharing activity is so poorly undertaken in modern organisations even though the technological capability to support it may be there.