The last decade quality of life (QOL) has been a central concept in the research about people with intellectual disabilities (ID). There are, however, still a lot of challenges to overcome. One such challenge is the measurement of the subjective well-being (SWB) - which is one of the two components of QOL - of people with severe and profound ID who communicate at a non-symbolic level. Therefore, in this dissertation we wanted to investigate how the SWB of this population could be measured. The most used method in the literature to measure the SWB of people with severe or profound ID is asking proxies to give estimations of the life satisfaction of the person. We, however, argued that measures of the SWB of people with severe or profound ID should focus on their positive and negative affect and not on their satisfaction. Until now there have been no studies investigating their affect, therefore in our first manuscript we tried to get a first view of this affect and the factors influencing it. To this end, we investigated the mood of 360 persons with severe or profound ID and examined which setting and person characteristics influenced their mood. We found that the mood of persons with profound ID was lower than the mood of persons with severe, mild or no disability. There were only person characteristics associated with their mood. In the next three manuscripts, we further investigated how the affect of people with severe and profound ID can be measured in a valid and reliable manner. Since affect consists of both moods and emotions, it is important to have valid measures for both the moods and emotions of people with severe or profound ID. However, the most used measure of emotions in this population, observation, has not been validated against a second source of information pertaining to emotions. Therefore, in our second manuscript, we validated the observations of emotions against the physiology of emotions. We presented 27 persons with severe or profound ID with 4 positive and 4 negative stimuli, videotaped the situation and recorded their respiration and heart rate variability. Each behaviour of the participant in the videotapes was then coded in terms of the expression of emotions. We found the same relationship between the emotion codes and the respiration and heart rate variability as would be expected from the results found in the population of persons without disability. This gives evidence for the validity of using behavioural observations of emotions. In our third manuscript we investigated if observations of mood and emotions could be discerned and how they were related in people with severe or profound ID. We presented 27 persons with severe and profound ID with 4 positive and 4 negative stimuli and videotaped the situation. We also asked a member of the direct support staff to give information about the mood of the person. We found that participants with high mood expressed more positive emotions than participants with low mood. There was no relationship between mood and the frequency of negative emotions or the intensity of negative or positive emotions. Although we have validated the observations of emotions against a second source of emotions, we think that, since self-reports are not available, it is important to include multiple sources of information to get a good view on the emotions of people with severe and profound ID. Therefore in our fourth manuscript we investigated if the physiology of people with severe and profound ID could give us information about the valence of their emotions independently from the observations of their emotions. We presented 27 persons with severe and profound ID with 4 positive and negative stimuli and measured their heart rate, skin temperature and skin conductance. We found that the heart rate for the negative stimuli was lower than the heart rate for the positive stimuli in the 4th to 6th second of stimulus presentation. This provides evidence that the heart rate can give information about the valence of the emotion independently from the observations of emotions. In our fifth manuscript we further examined the results of the fourth study by investigating the role of attention in the onset and regulation of emotions of people with severe or profound ID. Starting from hypotheses derived from the literature of people without disabilities, we examined if our participants were more attentive when presented with negative stimuli than with the positive stimuli in the first 4 to 6 seconds of stimulus presentation and if they had less attention to the negative stimuli after this period. We did not find more attention when presented with negative stimuli in the 4th to 6th second but did find less attention to the negative stimuli than to the positive stimuli after this period, which indicates that people with severe or profound ID possibly withdraw their attention to regulate their emotions. In our last chapter we discuss the methodological, theoretical and practical relevance of the research.