Breast cancer is a reality for 5000 women in Switzerland every year and, when these women are in a committed relationship, represents a critical event for their partners as well. For this reason, there is now a consensus in the literature to consider breast cancer as an event involving couples as much as women alone. Research should also take into account women and partners and should be extended to couples as a unit. Up to now, it has been difficult to draw a synthesis from the data available in the body of writings on the impact of cancer on couples, as methodologies have been diverse and vary between studies. Thus, the goal of this chapter is first of all to present the advantages and limitations of the two main methods of assessment that are used in the domain of couple relationships: questionnaires and observation. The necessity to combine the different methods is then discussed. At last, comments will be illustrated with a project conducted in Switzerland funded by the Swiss National Science Foundation that formed part of the National Centre of Competence in Research (NCCR) "LIVES - Overcoming Vulnerability: Life Course Perspectives"˝ and which examined couples facing breast cancer using a mixed method approach.