Aging is associated with an increased risk of cognitive impairment, such as Alzheimer's disease, vascular dementia and frontotemporal dementia. These diseases not only impair brain tissues and the nervous system, but also affect patients' verbal and non-verbal communicative ability. It is difficult, for instance, to assess pain in the cognitively impaired elderly because pain perception draws heavily on a patient's subjective interpretation and tolerance level. To seek an effective solution for pain detection in patients with dementia is therefore essential. From a systematic literature review covering the past decade, this paper presents a synopsis of the difficulty of pain assessment in the elderly with cognitive impairment, changes in the academic concept of pain, and explores factors resulting in obstacles to pain recognition. The obstacles to pain assessment derive mainly from three general factors. Firstly, classic definitions of early stage pain are inappropriate when applied to the elderly with cognitive impairment. Secondly, pain indicators are also unclear for this population. Thirdly, pain instruments and informants are ineffective in evaluating elderly patients with varying cognitive levels. To address these obstacles, a sound, multifaceted model of pain assessment for elderly patients with different severities of cognitive impairment is presented. On the basis of the above literature review, multiple methods for detecting aspects of pain in elderly people with cognitive impairment is recommended. To test the feasibility of the multidimensional model of pain assessment, further study is needed.