Background: Clinical pharmacy services play a key role in optimising medicines and improving patient safety. They aim for each patient to be reviewed daily by clinical pharmacists. However, current financial pressures within the NHS mean that there may not be resources for daily review of patients. Therefore, the ability to accurately screen and identify those patients with more urgent and/ or complex needs would be of great benefit, enabling appropriate allocation of costly staff resources. There have been attempts within some United Kingdom (UK) hospitals to implement locally developed pharmaceutical care screening tools, but these have not been methodically formulated for routine use and there is a lack of agreement as to what such tools should comprise. Thus, the overall aim of this study is to develop a screening tool that can be used by the hospital pharmacy team to triage new patient admissions according to the complexity of their pharmaceutical needs. It will mean that the right pharmacists see the right patients at the right time to deliver high quality and efficient pharmaceutical care to improve patient safety. Methods: This thesis contains three studies, each applying different methods. The first was a systematic review of the literature to provide a structured overview and description of existing assessment tools used by hospital pharmacy services to assess patient priority and/or complexity. Study Two obtained expert consensus on the design of a pharmaceutical care complexity screening tool for use on admission to hospital. Two Delphi studies were conducted to obtain consensus on the necessary components of a pharmaceutical care complexity screening tool and the appropriate frequency and competency of clinical pharmacist input for each complexity level. Identified and refined tool components from Study One and from our previous UK survey and interview study of prioritisation tools were included in the first two-round Delphi study. The Delphi One survey was distributed to international medication safety experts including hospital pharmacists, academics and physicians. The expert panel was asked to rank each component on importance, using a nine-point Likert-scale. In Delphi study Two, a framework analysis of the previous interview data led to the formation of statements relating to practical aspects of the tool. Clinical pharmacists with a management role identified from Delphi One and via professional networks were invited to complete the Delphi Two survey. The same rating process and consensus approach was used as in Delphi One. Decisions made by the Delphi panels were incorporated into the final version of the Adult Complexity Tool for Pharmaceutical Care (ACTPC). Study Three was a feasibility study conducted on the Acute Medical Unit (AMU) of three UK hospitals. It aimed to assess the practicality and feasibility of the ACTPC tool and identify the most efficient and effective ways to measure the impact of the tool on patient outcomes and pharmacist workload patterns. Results: The systemic review revealed that there has been growing interest in the development of pharmacy prioritisation tools in recent years. Seventeen published papers including eight UK based tools and nine international tools have described screening tools designed and used in clinical pharmacy services for the assessment of patients to identify high risk patients and guide pharmaceutical care. Over 300 components were extracted from the interview data and systematic review and then refined for inclusion in the first Delphi study. Thirty-three experts completed Delphi one and consensus was reached on 92 components. Components were then grouped and shortened to 33 items (e.g. all individual high-risk medicines were grouped into a high-risk medicine category). The final items were included in the first draft of the Adult Complexity Tool for Pharmaceutical Care (ACTPC), which stratified patients into three levels - highly, moderately or least complex. The second Delphi study was completed by 40 expert panel members and consensus reached on clinical appropriateness and feasibility of review frequency and experience of pharmacy practitioner to care for patients at each level. These decisions were incorporated into the final version of the ACTPC. The feasibility study tested multiple outcomes for evaluating tool effectiveness and validity. Despite being a feasibility study, our results showed a statistically significant reduction in the number of patients receiving doses of erroneous medications and the number of patients who had serious and minor prescribing errors. The data demonstrated tool validity as patients with high and moderate complexity levels had higher numbers of serious and significant prescribing errors, including missing doses of time critical medicines, therefore identifying those patients at greater risk of drug related problems. Conclusion: This work systematically developed a comprehensive pharmaceutical care complexity screening tool based on robustly collected data with input from national and international experts. ACTPC was found practical and feasible across three hospital sites. The feasibility study recommends that the ACTPC is tested in a larger study for its effectiveness in reducing actual patient harm and improving the delivery of patient centred pharmaceutical care. Further research is warranted to explore the use of the tool beyond AMU, the use of technology to deliver ACTPC and how the technician workforce can be utilised to prioritise patients.