Background: Frailty is the most problematic syndrome among older people. It is multifactorial and characterised by decreased reserve and resistance against internal and/or external stressors. Frail patients are at increased risk of adverse drug events, polypharmacy, potentially inappropriate prescribing, falls, unplanned and extended hospitalisations and mortality. To reduce the risk of these complications, prescribing in this vulnerable population should be driven by a patient-centred decision-making approach and a comprehensive medication review is warranted, including patient assessment and subsequent selection of the optimal treatment regimen. Pharmacists, as medication experts, play an essential role in optimising and ensuring the rational use of medications and reducing the risk of medication-related harm, in collaboration with other healthcare professionals (HCPs). However, the implementation of pharmacy services among frail older patients is not yet well established in acute care settings. To date, little work has been conducted to investigate the appropriateness, effectiveness and safety of medication use in hospitalised older frail patients. Additionally, there is limited evidence on the impact of medicines optimisation and the integration of a pharmacist-led medicines optimisation service on quality of prescribing as well as clinical and humanistic outcomes for this vulnerable population. Aim: This research aims to comprehensively study medication use among older frail patients, and the impact of medicines optimisation interventions and the involvement of pharmacists on patient clinical and humanistic outcomes in the acute care setting. Furthermore, it aims to explore HCPs' perspectives of a pharmacist-led medicines optimisation service for frail older patients in the acute care setting. Method: Qualitative and quantitative methodologies were used throughout the four studies described in this thesis. A systematic review was undertaken to determine the effectiveness of medicines optimisation interventions in improving medication appropriateness and clinical and humanistic outcomes in frail older patients in secondary/acute care settings. A service evaluation study was undertaken to evaluate the impact of the Medicines Optimisation in Older People [MOOP] model, a pharmacist-led case management approach, on outcomes and prescribing appropriateness for frail older people admitted to an acute frailty ward. Semi-structured interviews were conducted via telephone and Microsoft Teams with HCPs to explore their experiences and perspectives of the MOOP model. Data saturation was the main determinant of the sample size and thematic data analysis was conducted. Finally, three iterative rounds of a Delphi consensus validation online survey were undertaken to develop and validate a medication-related fall (MRF) screening and scoring tool developed from a tool used in current clinical practice and clinician input. Results: Based on the findings from the systematic review, medicines optimisation may improve medication appropriateness in frail older inpatients. However, the review highlighted the paucity of high-quality evidence examining the impact of medicines optimisation and the involvement of pharmacists on quality of prescribing as well as clinical and humanistic outcomes for frail older inpatients. The service evaluation study of the MOOP service revealed that for the 200 patients recruited into the service, 1020 interventions were made during and post hospitalisation; of these, 92% were assessed as significant and resulted in enhanced care standards. The service also resulted in improvement in prescribing appropriateness, as demonstrated by statistically significant reductions (P-value < 0.05) in Anticholinergic Cognitive Burden (ACB), Anticholinergic Effect on Cognition (AEC) and Medication Appropriateness Index (MAI) scores, falls risk score and total number of prescribed medications. In addition, the MOOP service demonstrated cost savings and reduced the annual medication cost. Semi-structured interviews with HCPs revealed four core themes: the complex patient population; the impact of the service; the key role of the pharmacist in the multidisciplinary team and resource requirements. The MOOP service streamlined and optimised care provision and the discharge process, improved HCPs' knowledge of medication and reduced their workloads, and increased patients' awareness of their medications. Participants acknowledged the MOOP pharmacist as a medication expert and an integral member of the multidisciplinary team. They highlighted the need for funding and staffing resources to support the implementation of the MOOP service to ensure continual delivery of optimal outcomes. Consensus was reached for 19 medications/medication classes to be included in the final version of the MRF tool and to reject eight medications/medication classes. Consensus was not reached regarding eight medications which were not included in the final version. Of the 19 included medications/medication classes, ten were classified as high-risk, eight as moderate-risk and one as low-risk in their potential to cause falls. To qualify and score the falls risk associated with the use of medications, numerical values are allocated to each medication class, with three points allocated to the high-risk medications; two points allocated to the moderate-risk medications and one point allocated to the low-risk medications. The overall medication-related falls risk for the patient is then determined by summing these scores. Conclusion: Medicines optimisation is safe, feasible and effective in improving prescribing appropriateness for frail older patients in acute care settings. A pharmacist-led medicines optimisation service was positively perceived by HCPs who were satisfied with their collaboration with clinical pharmacists in this service which enhanced provision of care and drug therapy, as well as knowledge and prescribing skills. The MRF tool is simple and has the potential to be integrated into medicines optimisation to reduce falls risk and negative fall-related outcomes.