In decision-making processes, competing knowledge claims create tensions, contestations, and negotiations between various social actors in their efforts to reach a decision. While attention to questions of knowledge (such as how certain knowledge gains legitimacy and authority) are useful in examining the dimensions and stakes of knowledge contestations, broadening the analysis to consider the complex role of unknowns can provide fruitful and nuanced insights into these contestations. Through a qualitative methodological research design, in this dissertation I focus on knowledge contestations in relation to the challenges of recognizing occupational diseases in the context of the Ontario workers' compensation system. The research questions that drive this investigation are as follows: (1) how do unknowns complicate knowledge contestations, specifically those surrounding the recognition of occupational diseases; (2) how do various types of knowledges and unknowns become mobilized in these recognition processes; (3) what counts as evidence in recognition processes, and what role does evidence play in supporting various knowledge claims; and (4) how do social and political factors influence the recognition of occupational disease? In exploring these questions, I primarily draw on three theoretical resources: new materialism, sociology of knowledge, and ignorance studies. I find that multiple dimensions of unknowns play a pivotal role in knowledge contestations over occupational disease recognition. The forms that such unknowns tend to take complicate and obscure connections between occupational factors and the development of disease. The mobilization of unknowns in contestations over disease recognition presents further challenges due to conflicting economic and other interests of the various social actors involved in these decision-making processes, as well as the broader influence of the dominant biomedical model in knowledge about disease and the body.