Some populations/groups are defined as hard to survey (H2S) in the research literature because of difficulty sampling the population, accessing the population, contacting members of the population, persuading them to participate, and interviewing group members (see Tourangeau, Chapter 1 in this volume). Reviewing the literature on populations referred to as "hard to survey" indicates the types of studies undertaken with these populations, the survey error challenges, and the methodological solutions used by researchers who study such populations. The H2S literature, however, does have limitations and research gaps exist.The problems that complicate obtaining high-quality data from H2S populations are diverse and often interrelated. This review uses a quality assurance and quality control framework to discuss the total survey error associated with H2S studies. The discussion is based on an extensive systematic review of the quantitative and qualitative literature related to H2S, as well as our own experiences surveying these populations. (For a discussion and definition of kinds of H2S populations and the characteristics that earn them the H2S label, see Chapter 1 in this volume.)We identify literature from various disciplines, relate these to the H2S challenges, and note which solutions are proposed or criticized. In the overview of challenges and practices, we also identify research gaps and general limitations that need to be addressed. Here, we consider types of error and survey process stages less frequently discussed or for which concrete data and solutions are sparse. We identify pertinent quality assurance/control steps to address such gaps and point to tools to remedy important measurement and documentation deficits. We hope that our systematic review can generate improved practices for surveying H2S populations.Our literature review on H2S populations entailed searching keywords related to the topic in academic search databases, including Academic Search Premier/EBSCO, JSTOR, and Five survey research and methodology students and three survey research and methodology professors and professionals undertook an appraisal process for this chapter. The students were