Purpose: The increase in drug prescription for the elderly raises the risk of the occurrence of potentially inappropriate medications (PIMs), thus increasing the incidence of drug-related problems. Likewise, potential prescribing omissions (PPOs) are also highly prevalent in the elderly. This study aimed at assessing the prevalence of PIMs in the elderly by using the EU (7)-PIM list, STOPP criteria version 2 and the Beers criteria version 2015, as well as the prevalence of PPOs by applying the START criteria version 2 in elderly nursing home residents and outpatients of the Eastern Central Region of Portugal. Patients and Methods: A descriptive cross-sectional study was carried out in a sample of 90 Portuguese elderly people. Age, gender, diagnoses and medication history were collected from the patients' clinical records. The prevalence of PIMs and PPOs was measured according to each of the criteria applied. Results: The patients' ages ranged from 65 to 103 years, with an average age of 84.15 years. In addition, the average number of medications prescribed was 7.6. The STOPP criteria identified 250 PIMs affecting 77 patients (85.5%), the EU(7)-PIM list detected 94 PIMs in 58 patients (64.4%) and the Beers criteria identified 69 PIMs in 51 patients (56.6%). Therefore, the STOPP criteria version 2 identified substantially more PIMs than the other two tools. Furthermore, by applying the START criteria 68 PPOs were detected in 52 patients (57.7%). Conclusion: A high prevalence of PIMs and PPOs was observed, suggesting the need to implement actions aimed at reducing the phenomenon and thus help to improve the quality of care provided in nursing homes. The variations in prevalence with the different tools suggest the need to carefully choose the tool for medication review in the elderly.