  Context: Grey Literature (GL) recently has grown in Software Engineering (SE)
research since the increased use of online communication channels by software
engineers. However, there is still a limited understanding of how SE research
is taking advantage of GL. Objective: This research aimed to understand how SE
researchers use GL in their secondary studies. Method: We conducted a tertiary
study of studies published between 2011 and 2018 in high-quality software
engineering conferences and journals. We then applied qualitative and
quantitative analysis to investigate 446 potential studies. Results: From the
446 selected studies, 126 studies cited GL but only 95 of those used GL to
answer a specific research question representing almost 21% of all the 446
secondary studies. Interestingly, we identified that few studies employed
specific search mechanisms and used additional criteria for assessing GL.
Moreover, by the time we conducted this research, 49% of the GL URLs are not
working anymore. Based on our findings, we discuss some challenges in using GL
and potential mitigation plans. Conclusion: In this paper, we summarized the
last 10 years of software engineering research that uses GL, showing that GL
has been essential for bringing practical new perspectives that are scarce in
traditional literature. By drawing the current landscape of use, we also raise
some awareness of related challenges (and strategies to deal with them).
