Multiple Sclerosis (MS) is a progressive and unpredictable disease that presents sufferers with multifaceted challenges in participation in everyday life. The main purpose of this dissertation was twofold: first, to explore the participation and perceived functioning of moderately and severely disabled people with MS in everyday life, and second to investigate the effect of a two-year multidisciplinary rehabilitation on everyday activities. This dissertation is based on data from a multidisciplinary group rehabilitation project for people with MS (n=113). Additional data (n=89) collected to evaluate the psychometric properties of the Finnish version of the Impact on Participation and Autonomy (IPA) questionnaire and predictors of participation using structural equation modelling (SEM). Participation and activities in everyday life were studied with the IPA questionnaire and the semi-structured interview Canadian Occupational Performance Measure (COPM), which are both self-assessment measures. The framework of the International Classification of Functioning, Disability and Health (ICF) was applied in the analysis. Improvement in performance in daily activities of the people with moderate and severe MS during the two-year multidisciplinary group rehabilitation was investigated with the COPM using repeated measures of analysis of variance. The results showed that the IPA is a valid and reliable measure that captures the autonomy aspect of participation of people with MS. In addition, perceived problems in participation and activities in daily life of people with MS supported the ICF comprehensive and brief core sets for MS. The results of the SEM indicated that quality of life and the psychological and physical impacts of the disease were the main predictors of participation and autonomy. The two-year multidisciplinary group rehabilitation improved performance and satisfaction with daily activities of people with both moderate and severe MS. The most noteworthy self-reported reasons for change during rehabilitation program were personal and environmental factors. Overall, the findings highlight both the diversity of experiences of participation and activities in everyday life of people with MS and that these experiences are not explained by disease severity. Therefore, self-assessment measures are recommended for measuring participation and activities in everyday life of people with MS. Moreover, the findings suggest that changes in performance and satisfaction in daily activities require long-term multidisciplinary rehabilitation.