Context: Software startups aim to develop innovative products, grow rapidly, and thus become important in the development of economy and jobs. Requirements engineering (RE) is a key process area in software development, but its effects on software startups are unclear. Objective: The main objective of this study was to explore how RE (elicitation, documentation, prioritization and validation) is used in software startups. Method: A multi-vocal literature review (MLR) was used to find scientific and gray literature. In addition, a case survey was employed to gather empirical data to reach this study's objective. Results: In the MLR, 36 primary articles were selected out of 28,643 articles. In the case survey, 80 respondents provided information about software startup cases across the globe. Data analysis revealed that during RE processes, internal sources (e.g., for source), analyses of similar products (e.g., elicitation), uses of informal notes (e.g., for documentation), values to customers, products and stakeholders (e.g., for prioritization) and internal reviews/prototypes (e.g., for validation) were the most used techniques. Conclusion: After an analysis of primary literature, it was concluded that research on this topic is still in early stages and more systematic research is needed. Furthermore, few topics were suggested for future research. © 2018 Elsevier Inc.