We investigate co-operative innovative activity in four major European countries, France, Germany, Spain and the UK, using internationally comparable firm-level data for manufacturing and service sectors. We examine the roles of knowledge flows, cost- and risk-sharing and public financial support in firms' decisions to collaborate. Our results suggest that firms which place greater value on external information flows are more likely to co-operate with the research base than with other firms and that firms facing appropriability problems are more likely to co-operate with the research base and with upstream and downstream firms than with direct competitors. We find evidence for Spain to suggest that firms collaborate to overcome risks and financial constraints. We also find that receipt of public support is positively related to undertaking collaborative innovation. In line with the focus of policy, this relationship is strongest for co-operation with the research base.