This paper details the rationale and methodology behind the construction of the Persistent and Common Component of Inflation (PCCI), a measure of underlying inflation in the euro area. The PCCI reflects the view that underlying inflation captures widespread developments across the Harmonised Index of Consumer Prices (HICP) basket and that it is the persistent component of inflation. Methodologically, it relies on a generalised dynamic factor model estimated on a large set of disaggregated HICP inflation rates for 12 euro area countries. For each individual inflation rate, we estimate a low-frequency common component, i.e. a component driven by shocks or factors that are relevant for all inflation series and capturing cycles longer than three years. The PCCI is a weighted average of these common components. It is an alternative to the typical exclusion-based measures used to gauge underlying inflation (e.g. HICP excluding food and energy), as it does not a priori exclude any HICP items. It exhibits a set of desirable properties as a measure of underlying inflation, and it is a good tracker of more lasting inflationary developments (judging by smoothness and bias). Furthermore, it is timely and signals turning points with some lead, while acting as an attractor for headline inflation.