Agile software development (ASD) methods have gained popularity in the industry and been the subject of an increasing amount of academic research. Although requirements engineering (RE) in ASD has been studied, the overall understanding of RE in ASD as a phenomenon is still weak. We conducted a mapping study of RE in ASD to review the scientific literature. 28 articles on the topic were identified and analyzed. The results indicate that the definition of agile RE is vague. The proposed benefits from agile RE included lower process overheads, a better requirements understanding, a reduced tendency to over allocate development resources, responsiveness to change, rapid delivery of value, and improved customer relationships. The problematic areas of agile RE were the use of customer representatives, the user story requirements format, the prioritization of requirements, growing technical debt, tacit requirements knowledge, and imprecise effort estimation. We also report proposed solutions to the identified problems.