We analyze whether the growing importance of passive investors has influenced the campaigns, tactics, and successes of activists. We find activists are more likely to pursue changes to corporate control (e.g., via board representation) and to forego more incremental changes to corporate policies (e.g., via shareholder proposals) when a larger share of the target company’s stock is held by passively managed mutual funds. Furthermore, higher passive ownership is associated with increased use of hostile, expensive tactics (e.g., proxy fights) and a higher likelihood the activist obtains board representation or the sale of the targeted company. Overall, our findings suggest that the increasingly large ownership stakes of passive institutional investors mitigate free-rider problems associated with certain forms of intervention, and ultimately increase the likelihood of success by activists.