Due to the profound impact that aphasia has on both the person with aphasia and their close family members, aphasia is a family problem. Aphasia is a communication disorder, most commonly caused by stroke. It is associated with impairments in spoken language, understanding, reading, and writing that impact upon daily activities, participation in society, and the quality of life of those with the condition and their family members. However, family-centred rehabilitation programs, policy, and funding are not well established. In the International Classification of Functioning, Disability and Health (ICF), the World Health Organization introduced the term “third-party disability” and identified the need for further investigation into family members’ functioning and disability in relation to a significant other’s health condition. In this thesis, the term “third-party functioning” is used to describe positive and/or neutral changes to a family member’s functioning as a consequence of a significant other’s health condition; “third-party disability” describes negative changes. The overall aims of this thesis are to: i) describe third-party functioning and disability in family members of people with aphasia post-stroke, and ii) develop the Significant other Scale for family members of people with Aphasia (SOS-Aphasia), a scale for measuring third-party functioning and third-party disability in this population. Two systematic reviews were conducted to provide a summary of the current literature related to family members’ third-party functioning and third-party disability secondary to aphasia. Positive/neutral and negative findings were extracted from included articles, then synthesized and mapped to the ICF. These reviews showed that family members experienced third-party functioning and third-party disability secondary to aphasia in the Body Functions and Activities and Participation components of the ICF, in addition to the development and exacerbation of health conditions. The results laid the groundwork for the qualitative-quantitative sequential mixed methods study that followed. In the first phase of the study, qualitative methods were used to explore the positive and negative effects of aphasia on family members, thus expanding and confirming our existing understanding of third-party functioning and third-party vii  disability. Twenty family members participated in individual in-depth semi-structured interviews, which were analysed using qualitative content analysis with research codes subsequently mapped to the ICF. The results revealed five categories of positive/neutral aphasia-related changes, including: (1) emotions (e.g., focusing on the positive); (2) communication (e.g., talking to the person with aphasia more); (3) relationships (e.g., making new friends); (4) recreational activities and social life (e.g., taking up new hobbies); and (5) paid/volunteer work or education (e.g., volunteering to help people with aphasia). In addition, seven categories that described the negative effects of aphasia on family members were revealed: (1) physical, mental, and emotional health (e.g., tired); (2) communication (e.g., difficulty communicating with the person with aphasia); (3) relationships (e.g., lack of physical intimacy between spouses); (4) recreational activities and social life (e.g., restricted social activities); (5) paid/volunteer work or education (e.g., took extended time off work); (6) domestic and caregiving responsibilities (e.g., transporting the person with aphasia places); and (7) finances (e.g., loss of person with aphasia’s income). Research codes were mapped to two domains within the Body Functions component (i.e., Mental functions and Functions of the digestive, metabolic and endocrine systems) and eight domains within Activities and Participation components of the ICF (i.e., Learning and applying knowledge, General tasks and demands, Communication, Self-care, Domestic life, Interpersonal interactions and relationships, Major life areas, and Community, social and civic life). Health conditions (e.g., depression and anxiety) associated with the aphasia of a significant other were also identified in family members. Mapping of the qualitative research codes to the ICF demonstrated how the ICF framework could be applied to family members. The second phase of the study involved the development and validation of the SOS-Aphasia. The SOS-Aphasia items were derived from the constructs of third- party functioning and third-party disability identified in the qualitative study and the response scale format was based on the ICF. Following the development of the SOS- Aphasia, 104 family members completed the original 34-item version. Factor analysis and Rasch analysis were used to examine the underlying structure and internal construct validity of the SOS-Aphasia. Test-retest reliability and feasibility were also investigated. Based on the analysis, five SOS-Aphasia subscales were identified and viii 10 items marked for deletion. The revised 24-item SOS-Aphasia demonstrated preliminary evidence of good psychometric properties. In summary, this series of studies demonstrates that family members experience third-party functioning and third-party disability, as well as changes to their health, secondary to a significant other’s aphasia. In addition, the SOS-Aphasia shows validity and reliability in measuring third-party functioning and third-party disability. Together, these findings provide the motivation for the inclusion of family members of people with aphasia in a family-centred care model with implications for research, practice, and policy.