Frailty is a geriatric syndrome characterized by age-related decrease in functional reserves and function, involving multiple organs and systems, resulting in amplified vulnerability to stressors, and an increased risk of adverse outcomes. Diagnosing frailty informs about a person’s general health and the hazards when exposed to disease, invasive diagnostic or therapeutic procedure. To diagnose frailty instruments were developed, tailored to different patient populations and different purposes: in screening for frailty, patient triage, assessment of perioperative risk, care planning, avoidance of deconditioning during illness. In being fitted to population and clinical setting, a frailty instrument is valuable under the defined circumstances; it may be inadequate if applied to different settings. Based on the vast literature and personal experience in practice, we outline positive matchings between diagnostic purpose and frailty instruments selected out of the more than 50 existents. There is given emphasis on generally accepted guidelines for frailty diagnosis and management, as well as on recent developments proposing molecular chemistry and artificial intelligence in diagnosing and monitoring frailty.