The purpose of this iHelp deliverable, D3.5 “Secondary data capture and interoperability I”, a public report, is to act as an inventory of secondary data for the iHelp clinical studies, ensuring that all clinical partners know what has been made available for extracting information from their end-users, mostly patients, but in cases of preventive studies, also the general public. No matter their status (patient, or citizen), end-users in iHelp are study participants, and this is how they are referred to in this document. The deliverable also documents the choices made on what to measure and what to simply ask the study participants to provide, trying to strike a balance between loading people with devices (and cost) but also asking too much of them in terms of data entry. In that respect it is of interest outside the iHelp community, serving as a data collection manual outside of the strict clinical setting. This document is delivered in M10 (October 2021), between the two phases of requirement collection. It follows up on the early requirements reported in D2.1, “State of the art and requirements analysis I” in M6 and the discussions between the partners responsible for secondary data collection and the clinical partners. As a result, a mature secondary data inventory has been drafted. It is followed by D2.2, “State of the art and requirements analysis II” in M12, where the final requirements will be documented, possibly leading to some changes in the inventory. Any such changes will be documented in the second version of this deliverable, D3.6 “Secondary data capture and interoperability II”, due in M22 (October 2022).