The overarching theme of this thesis is the prevention of progressive-type disability. Unlike catastrophic disability, progressive disability is gradual and more common in older adults. Because progressive disability can take years to develop, it is often conceptualized as a continuum, from less to more disabled. Disability prevention, by definition, is designed to identify people who are as yet nondisabled but at high risk for future functional decline by identifying an early functional state associated with increased risk of subsequent disability (Fried & Guralnik, 1997). This thesis sought to address two challenges associated with identifying an early functional state of disability. The first challenge relates to instrument calibration. Traditional instruments (based on self-report) used for assessing disability, scales of activities of daily living (ADL) and instrumental activities of daily living (IADL), were originally developed to describe levels of functional status in institutionalized older adults. Thus, these instruments poorly discriminate, as well as underestimate disability in the early stages of development. Poor discrimination refers to tasks or activities (i.e., scale items) that prove unresponsive to changes in a particular person’s ability level. Performance measures on the other hand, such as walk time or grip strength, have proven to be quite responsive to early declines in functional status. Despite the popularity of performance measures used to assess health status in epidemiology or gerontological research, evidence suggests that they measure a somewhat different construct than self-reported activities of daily living. ADLs have a long history of use in the medical community, yet it has been proposed that the relative standing of ADLs, in relation to communitydwelling older adults, could be enhanced by improving construct validities that are at least equivalent to those of physical performance measures. Item response theory (IRT) methodology can be used to improve the structure of ADL scales so that they are more sensitive in detecting the early stages of functional decline within relatively high functioning older adults; a stage that has been shown to be more responsive to clinical interventions aimed at prevention of overt disability or frailty. IRT can improve ADL scales in multiple ways: by confirming an underlying uni-dimensional continuum of disability, establishing interval level measurement or item hierarchies, and increasing scale precision. As part of this thesis I conducted a systematic review of functional status scales, applied to community-dwelling older adults, which employed IRT procedures. The review was useful in that it draws attention to areas of functional assessment that can be improved upon, most notably, the topic of establishing interval level data and construct under-representation. Using data from the Cardiovascular Health Study, I was able to show that a common hierarchy of functional decline was observed for a diverse set of conditions and diseases that are prevalent among community-dwelling older adults. Such an indicator could be used to identify hierarchical declines relating to severity in diverse patient populations. Improvements in validity of functional status scales can also lead to the use of ADL-IADLs as potential determinates of disability, rather than simply acting as outcome measures of disability. Again using data from the Cardiovascular Health Study, I examined the predictive power of IADL (mobility-type) items on later disability. Self reported difficulty in 2 or 3 of the most difficult IADL items increased the odds of being disabled eight years later by a factor of 3.5. The odds of being disabled fell to 1.9 for those reporting difficulty with one item. The second challenge of this thesis relates to defining determinants of functional decline that manifest themselves at the earliest stages of the disablement process. As previously stated physical performance measures have been shown to be sensitive to early stages of functional decline. However, can other measures, potentially spanning multiple domains, be used to identify those at high risk for future disability? In particular I was interested in whether psychosocial and cognitive variables could be used to detect changes in functional status at the preclinical stages of the disablement process. With regard to the Cardiovascular Health Study, I was able to show that, for subjects within the normal range of cognitive functioning, performance in the lowest quartile of the Digit Symbol Substitution Test resulted in a 2.2 increase in the odds of being disabled. Performance on this measure, as well as selfreported mobility noted above, could detect decrements in functional status as much as 8 years prior. With the use of the Lothian Birth Cohort sample I explicitly investigated the psychosocial domain. I found that the level of depressive symptoms increased the odds of being disabled by 56%. Again, these symptoms were assessed as much as eight years prior to self-reported disability. The general findings of this thesis indicate that refinements in ADL-IADL measures can aid in the detection of disability at the pre-clinical level, and that cognitive function and intra-individual factors play a pivotal role in speeding up or slowing down the disablement process.