Problematic painkiller use is a large and increasing problem worldwide, leading to serious physical, psychological and social consequences. Existing research indicates that accessibility of painkillers and psychological factors could have a role in problematic painkiller use. To clarify, accessibility refers to ease of obtaining painkillers, whilst psychological factors refer to individual-level processes and meanings that influence mental states (Upton, 2013). However, there have been few studies conducted, and these studies have focused mainly on either clinical samples or women with childbirth pain. Hence, the role among the general population is less clear. The aim of the present thesis was therefore to focus on the role of accessibility and psychological factors in problematic painkiller use among the general population. Three studies were conducted: one study compared the general population of the UK (N = 295) and Egypt (N = 420) regarding the role of accessibility of painkillers and psychological factors, including attitudes and beliefs towards pain, painkillers, self-medication and alternative methods of pain relief; another study was a multi-national comparison of these variables among the general population of more countries including Germany (N = 217), USA (N = 146), Australia (N = 93) and China (N = 76); another study focused on the role of psychological factors over time among the UK general population (N = 529), specifically attitudes and beliefs towards painkillers, as well as pain catastrophising, pain acceptance, pain self-efficacy and alexithymia. In these studies, the role of accessibility and psychological factors was investigated using online surveys, with participants aged 18 years or over, who experienced pain in the last month, used over-the-counter (OTC) or prescription painkillers in the last month, and were residents of the countries concerned. An additional study was conducted to develop 14-item versions of the Survey of Pain Attitudes-Brief (SOPA-B-14) and the Pain Medication Attitudes Questionnaire (PMAQ-14), which also tested the validity of these scales. Results Accessibility of painkillers and psychological factors predicted problematic painkiller use. However, there were several differences between the countries regarding the particular role of these factors. In the longitudinal study of the UK general population, changes in psychological factors were found over time, but attitudes and beliefs about withdrawal from painkillers was the only psychological factor that predicted problematic painkiller use over time. In addition, testing the validity of the SOPA-B-14 and PMAQ-14 showed that these scales were valid. The present research provided understanding regarding the role of accessibility and psychological factors in problematic painkiller use among the general population, and the role of psychological factors over time. Based on this understanding, interventions focusing on accessibility and psychological factors should be developed to reduce problematic painkiller use, but tailored to the particular factors that were predictors for each country. The present research also developed a valid SOPA-B-14 and PMAQ-14, therefore these scales can be used rather than the full versions to make assessment easier.