Private land comprises over half the land mass of the United Statesdominating certain ecosystems and hosting large numbers of threatened and endangered species. Understanding privately owned properties is thus critically important to conservation, yet these lands remain understudied by conservation biologists. A key factor in this lack of research is the difficulty of gaining permission to access private lands. However, there has been almost no empirical work to offer guidance for natural scientists on this issue. Using a combination of mail surveys and interviews, we undertook one of the first empirical studies of landowner decisions regarding access to their private property for research, and we identified the primary drivers and deterrents. We found that natural scientists may have more success gaining access if the landowners are interested in learning about the research taking place, if they have previously allowed research on their property, if they have positive attitudes toward conservation, and if they have larger properties. We also found that many landowners allowed research out of a desire to be helpful. Conversely, landowners are less likely to allow research if they are concerned that doing so will restrict free use of their property. Landowner age, education, trust in sci-