We use a model of combined endogenous growth and economic geography to study the impact of regional economic integration on the member and non-member countries of a regional union. Regional integration affects growth through interregional technology diffusion symbolized by knowledge spillovers generated at home and spreading to the partner countries. Spillovers flow from the leader to the follower. Following integration, the lagging country has access to a bigger stock of knowledge that fosters an increase in its rate of growth and extends the diversity of its products. Trade in goods - or in FDI - and flows of ideas are two faces of the same coin. We show that the progressive decrease in transaction costs through the phasing out of barriers to trade together with product imitation can foster growth and convergence in the member countries. However, in order to avoid eventual trade and investment diversions, the non-member should envisage to join the integrated zone.