I expound a tractable model of an infinite-horizon multi-sector econom
y where endogenous growth in the aggregate results from innovation rac
es which go on in each and every sector. I identify five externalities
that have strategic implications: the 'contemporaneous real income',
the 'lagged real income', the 'aggregate demand', the 'Shleifer' and t
he 'competition' effects. I prove that multiple stationary equilibria
can exist and lead to different rates of growth, and show that, in any
stationary equilibrium, the rate of growth is constant. Finally, I pr
ove that the rate of growth may be higher or lower than social welfare
warrants.