This article describes the design and theoretical foundations of the Pool Cool Diffusion Trial and reports 1st-year findings. Aims of the study are to evaluate the effects of 2 strategies for diffusion of the Pool Cool sun safety program on implementation, maintenance, and sustainability; improvements in environmental supports for sun safety in swimming pools; and sun protection habits and sunburn among participating children. There was a high rate of program participation (86.6%; n = 375 swimming pools) in the 1st year and somewhat lower study participation (75.8%). Analysis of pool manager surveys revealed a time effect for overall sun safety programs and for sun safety policies, environmental strategies, and programs for pool users. There were few differences in implementation between treatment groups in year one.