This dissertation examines various internal factors within a county and external factors outside the county that influence policy adoption. Particular attention is given to external factors, or the possible ways that counties respond to the policies of their neighbors. This study focuses on the multiple external processes involved that contribute to policy adoption. These include policy diffusion, lagged diffusion, and conditional diffusion. These processes are used to test hypotheses that will better explain how counties react to their neighbors' policy adoptions. Unlike most studies that examine only one process, multiple processes are tested to determine if they vary across different policies. This study explores Tax Incremental Financing districts, smoking bans, and the sales tax rates of counties in the state of Missouri. Missouri is politically, economically, and demographically similar to other states, therefore making it a sufficient case for this study. I find that different processes are involved in policy adoption, and that counties mainly react to one another through economic competition. I also find that certain county-level characteristics influence a county's likelihood of adopting policies. These findings will enable scholars to better determine the multiple processes involved in county policy adoption.