The aim of this article was to ascertain nursing home residents' preconceptions about institutionalisation and analyse the causes and circumstances of and the justification for their admission. Grounded theory was used to design and analyse a qualitative study based on in-depth interviews in a theoretical sampling of 20 persons aged over 65 years with no cognitive impairment, and eight proxy informants of residents with cognitive impairment, institutionalised at a public nursing home in Spain. Our analysis revealed that preconceptions about nursing homes differ between residents and relatives, and are strongly influenced by the views held by society about such centres and by previous experiences. Regarding the causes and circumstances underlying nursing home placement, while the principal cause of institutionalisation among residents with cognitive impairment was the ineffectiveness of informal care systems, in the case of residents without cognitive impairment reasons tended to revolve around two main themes: social causes (loneliness, not be a burden to the others, household-related, comfort and absence of relatives in the vicinity), and limitations in physical functioning, with the former predominating. This study shows society's perception of such centres and the circumstances surrounding admission. These points of view are useful for analysing why informal care systems prove inadequate, and are crucial for designing programmes targeted at acceptance and successful adaptation to institutionalisation when this becomes necessary.