In my dissertation, I examine the impact of ambiguity (Knightian uncertainty), alongside that of risk, on firms’ voluntary disclosure decisions. I confirm the well-known result that an increase in risk—uncertainty over outcomes—is associated with an increase in management guidance (earnings and capital expenditure forecasts). Conversely, I find that an increase in ambiguity—uncertainty over the probabilities of outcomes—is associated with less guidance. Furthermore, I show that ambiguity decreases following voluntary disclosures, consistent with managers being aware of and reacting to heightened ambiguity. Finally, I provide novel empirical evidence showing that guidance under ambiguity has adverse capital market consequences. Even though the ways through which risk impacts managers’ disclosure decisions have been extensively studied in the accounting literature, no extant research has examined whether and how ambiguity impacts these decisions. My findings are consistent with the notion that managers’ take into account the ambiguity in the environment, showing that ambiguity has an important and distinct impact on their voluntary disclosure decisions.