This study adopts an institutional theory perspective of the adoption of ERP (Enterprise Resource Planning) systems by medium sized firms in India. The study contributes to the understanding of the operation of the three isomorphic pressures: coercive, mimetic and normative, in the adoption of IS by moving beyond the common view of these as 'monolithic entities'. The study is undertaken by means of nine qualitative case studies. The research makes three contributions. Firstly, the study shows the complete range of mechanisms by which the three isomorphic pressures act within and across organisations. Secondly, our findings contribute to aiding the conceptual clarity of parts of institutional theory, by explicating areas of overlap and coalescence of the three macro-level pressures. Finally, our elucidation of processes and mechanisms are of value to practitioners and policy makers, since it is at the level of these processes and mechanisms that individuals and groups can most readily and easily take action.