We examine cost-reducing investment in vertically-related oligopolies, where firms may be vertically integrated or separated. Analyzing a standard linear Cournot model, we show that: (i) Integrated firms invest more than separated competitors. (ii) Vertical integration increases own investment and decreases competitor investment. (iii) Firms may integrate strategically so as to preempt investments by competitors. Adopting a reduced-form approach, we identify demand/mark-up complementarities in the product market as the driving force for these results. We show that our results generalize naturally beyond the Cournot example, and we discuss policy implications.