Multiple Sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system leading to progressive impairment of various neurological functions (Kesslering et al, 2002). It is the commonest cause of chronic neurological disability among young adults in the western world (Me Donnell et al, 2001). The physical and psychological effects of multiple sclerosis can impact heavily on any of these important life activities and so people with multiple sclerosis are more likely to be unemployed than the general population (Solari et al, 2001). Premature unemployment can have significant financial and social consequences in adulthood. Like the majority of the general population, many of those who have multiple sclerosis will still have dependants and regular financial commitments in their forties and fifties, such as a mortgage. Therefore, unemployment may be disastrous for the family, especially if it is the main wage earner who loses their job. Background: As well as the financial gain from work, it can also be an enjoyable activity that can define people and help them fulfil a social role. In recent years there has been more awareness of the burden of disease, not just physically but emotionally and financially and the disease and demographic factors contributing to job loss in the multiple sclerosis population have been identified. The effect of vocational rehabilitation in chronic diseases has also been explored to a degree and the Organisation for Economic Co-operation and Development (OECD) report in 2003 stated that early vocational rehabilitation intervention is the most effective measure against the dependence on benefits that is a consequence of long-term ill health (Frank, 2003). However at present there is no outcome measure to assess the need for intervention and it still remains to be seen if costly interventions, such as, the disease modifying therapies or vocational rehabilitation have any effect on the working life of those with multiple sclerosis. Work disability is premature work cessation due to a health problem or disability. Prior to work disability there may be a period of work instability. Work instability is a state in which the consequences of a mismatch between an individual’s functional abilities and the demands of his or her job can threaten continuing employment if not resolved. During this time the patient is most at risk of job loss and timely intervention in the work place can facilitate job retention (Gilworth et al, 2003).The aim of this thesis was to produce a valid and reliable Work Instability Scale (WIS) for multiple sclerosis in order to be able to predict those at risk of job loss. Methods: A multiple methodological approach, involving both qualitative and quantitative research techniques was used and a disease specific, work instability scale, the MS-WIS was developed and tested for appropriate psychometric properties. In depth, semi-structured interviews were conducted with people with MS in order to explore the issues of working while having MS. Qualitative analysis of these interviews using a grounded theory approach yielded themes that described the issues of working with MS. Further qualitative analysis of the interviews using content analysis methodology was being done alongside this which identified the items suitable for inclusion in the scale, a process known as item generation. The scale was then subjected to in-depth psychometric testing using Rasch analysis. Rasch analysis was chosen as this would ensure that a single construct was being measured and allowed the validity and the reliability of the scale to be tested. Results: Twenty seven people were interviewed for the qualitative study, fifteen men and twelve women and the interviews transcribed and studied. Four main themes emerged as factors contributing to work instability on qualitative analysis of the data: i) The impact upon work of the physical and cognitive aspects of the disease. ii) The extent to which the environment and organisational aspects of working life can affect job ability and retention. iii) The social aspects of the working environment. iv) The psychological aspects of working. Item generation initially yielded a scale consisting of one hundred and twenty two items. A postal questionnaire yielded 109 responses, the data from which then had to undergo in-depth Rasch analysis, during which items were discarded. Vocational assessment was used as a gold standard. This process resulted in a valid, reliable twenty one item scale - the Multiple Sclerosis Work Instability Scale (MS - WIS). The final scale is quick and easy to complete. It is scored in 3 bands indicating low, medium and high risk of Work Disability. The medium - risk threshold has 88% sensitivity and 60% specificity and the sensitivity and specificity of the scale reaches 100% at the high - risk threshold with regards to the need for work place intervention. Conclusion: MS has been shown to have a considerable impact on the working life of the individual. A disease specific outcome measure to quantify the risk of job loss in an individual with MS with good psychometric properties has been developed. The MS-WIS has been derived from a strong conceptual framework and a strong mathematical measurement model. It has been rigorously tested for its psychometric properties and has high sensitivity and specificity for risk of job loss.