Book excerpt: In this Research Topic the two editors bring together a series of articles that use facet theory and allied approaches to research. Since its inception in the work of Louis Guttman in the mid twentieth century, facet theory has become an established approach within social science research. In addition, over the past 70 years a wide range of research publications have appeared operating within the theoretical and analytic rubric of facet theory and for the last two decades a biennial international conference has been held devoted to facet theory research. When using a facet theory approach, an implicit aim of the research within this framework is to bring together in an explicit manner a clear definition of the content area that is being investigated along with data analysis procedures. Integrating the explicit design of research content (for example, attitudes, values, etc) and its subsequent analysis (for example to identify the variables that are influential to respondents in relation to the specific area under investigation) allows for the construction of theory relating to the content area and for the meaningful measurement of complex research areas. The clear explication of an area of research content, is achieved through the use of a mapping sentence (MS). In a MS all of the pertinent variables (called facets) associated with the specific subject matter of the research study are specified in the form of a natural language sentence where facets (variables) are arrange to demonstrate how these theoretically relate to each other. Sub-levels of facets are defined in such a way as to capture the relationships of research variables (facets) to each other and the overall research domain. Background variables are also stated in the MS along with a specified range over which observations will be made to test the veracity of the structural hypotheses (statements regarding the proposed manner in which variables are related to the study’s content) implicit in the MS. Furthermore, by using a MS the researcher is able to select variables that appropriately address the area of content. Traditionally, facet theory has been used in quantitative research but has recently been applied to the analysis of qualitative and philosophical research which incorporates a declarative mapping sentence in such research and which is included in this Research Topic. In order to interrogate these structural hypotheses, quantitative data analysis procedures are employed, such as Smallest Space Analysis (SSA) and Partial Order Scalogram Analysis by base Coordinates (POSAC). In SSA the structure of the content area of interest (the variables that have been included in the MS) can be interrogated as observations that have been made along the specified outcome range, are represented geometrically (as partitioned regions) in a concept-space related to the specific research domain. Individual respondents may also be investigated in terms of their profile of facet related scores using POSAC. A similar approach is employed when analysing information from qualitative facet theory research, which interrogates the structural hypotheses present in the declarative mapping sentence through approaches such as content and narrative analyses. This Research Topic presents work from scholars with particular emphasis upon how the approach has developed both theoretically and in terms of its application, new areas of application, and advances in theory development.