This study aims to i) To characterize the functional status and explore the determinants of functional status decline of people with IlD ii)To determine the measurement properties of functional status instruments in people with Interstitial lung diseases (ILD) iii) To identify the impact of ILD and the participants' perspectives on functional status through interviews iv) Explore the progression of functional status progression in people with ILD and v) Develop a multidimensional index, incorporating functional status parameters, to predict mortality in people with ILD. Patients with ILD will be recruited via the pulmonology services at hospitals, namely from Centro Hospitalar de Vila Nova de Gaia/Espinho (CHVNG/E), Centro Hospitalar do Baixo Vouga (CHBV) and Centro Hospitalar de Entre o Douro e Vouga (CHEDV). Sociodemographic, clinical characteristics (i.e., smoking habits, vital signs and symptoms), anthropometric (i.e., height and weight to compute body mass index) and general clinical data (i.e., medication, oxygen therapy, non-invasive ventilation, acute exacerbations, hospitalizations and number of hospital admissions in the last month and year, length of stay), as well as prior and follow-up spirometric measurements and arterial blood gas will be collected from clinical records for patients' characterization. Mortality and rehospitalizations will be explored during the study period. Peripheral muscle strength, functional status, daily physical activity, self-reported symptoms, functional status, impact of the disease and health-related quality of life. Qualitative data from interviews. The assessments will be conducted at 6 time points: baseline and 1 week after for instrument validation, followed by assessments every 6 months for 2 years. It is expected that: i) Functional status limitations can be comprehensively identified and measured in individuals with ILD. ii) Some measures are valid and reliable indicators of functional status in individuals with ILD. iii) Different profiles of functional status progression will be identified in individuals with ILD, including stable, slow, and fast decline. iv) A multidimensional index incorporating functional status will improve the accuracy of predicting mortality and outperform the predictive ability of the current GAP Index.