Karrikins are a small family of naturally occurring plant growth regulators present in the smoke and char produced from burning plant material in wildfires. They can stimulate germination of dormant seed and can influence seedling morphogenesis. Although Arabidopsis thaliana is not considered to be a smokeresponsive species, karrikins will stimulate seed germination under the appropriate circumstances and will cause repression of hypocotyl elongation in low light. This chapter describes how to conduct assays of the activity of karrikins on Arabidopsis seeds and seedlings. The methods presented can potentially be modified for use in a range of Arabidopsis genotypes or in other plant species.