Whilst science has a strong reliance on quantitative and experimental methods, there are many complex, socially based phenomena in HCI that cannot be easily quantified or experimentally manipulated or, for that matter, ethically researched with experiments. For example, the role of privacy in HCI is not obviously reduced to numbers and it would not be appropriate to limit a person's privacy in the name of research. In addition, technology is rapidly changing – just think of developments in mobile devices, tangible interfaces and so on – making it harder to abstract technology from the context of use if we are to study it effectively. Developments such as mediated social networking and the dispersal of technologies in ubiquitous computing also loosen the connection between technologies and work tasks that were the traditional cornerstone of HCI. Instead, complex interactions between technologies and ways of life are coming to the fore. Consequently, we frequently find that we do not know what the real HCI issues are before we start our research. This makes it hard, if not actually impossible, to define the variables necessary to do quantitative research, (see Chapter 2). Within HCI, there is also the recognition that the focus on tasks is not enough to design and implement an effective system. There is also a growing need to understand how usability issues are subjectively and collectively experienced and perceived by different user groups (Pace, 2004; Razavim and Iverson, 2006). This means identifying the users' emotional and social drives and perspectives; their motivations, expectations, trust, identity, social norms and so on. It also means relating these concepts to work practices, communities and organisational social structures as well as organisational, economic and political drivers. These issues are increasingly needed in the design, development and implementation of systems to be understood both in isolation and as a part of the whole. HCI researchers are therefore turning to more qualitative methods in order to deliver the research results that HCI needs.With qualitative research, the emphasis is not on measuring and producing numbers but instead on understanding the qualities of a particular technology and how people use it in their lives, how they think about it and how they feel about it. There are many varied approaches to qualitative research within the social sciences depending on what is being studied, how it can be studied and what the goals of the research are.Within HCI, though, grounded theory has been found to provide good insights that address well the issues raised above (Pace, 2004; Adams, Blandford and Lunt, 2005; Razavim and Iverson, 2006). The purpose of this chapter is to give an overview of how grounded theory works as a method. Quantitative research methods adopt measuring instruments and experimental manipulations that can be repeated by any researcher (at least in principle) and every effort is made to reduce the influence of the researcher on the researched, which is regarded as a source of bias or error. In contrast, in qualitative research, where the goal is understanding rather than measuring and manipulating, the subjectivity of the researcher is an essential part of the production of an interpretation. The chapter therefore discusses how the influence of the researcher can be ameliorated through the grounded theory methodology whilst also acknowledging the subjective input of the researcher through reflexivity. The chapter also presents a case study of how grounded theory was used in practice to study people's use and understanding of computer passwords and related security.