The quantitative knowledge of the trajectory of discrete elements immersed in fluids is an important data that can be used in many scientific and industrial fields. In the field of Building science and technology, the challenge is both environmental and energetic. On the environmental front, the issue is the control of indoor air quality, a public health issue. This involves predicting the motion of airborne contaminants or of dangerous gases in a confined space, in order to optimize ventilation or evacuation strategies. The current health crisis is a reminder of the importance of such a prediction. In terms of energy, knowing the trajectory of the air makes possible to understand the nature and dynamics of airflows and therefore to optimize heating or cooling systems dedicated to indoor thermal comfort. The numerical simulation tools (Computational Fluid Dynamics models) currently widely used to predict the flows motion, struggle to give satisfactory results except in very well-defined typical cases. Therefore, high quality experimental data is still a key step in the indoor airflow studies as well as in the bioreactor design, and plays a fundamental role in the validation and development of numerical models. However, the current measurement techniques are insufficient for the complete characterization of the flows involved. For example in buildings, airflows are mostly turbulent, low-velocity, strongly three-dimensional and large-scale, whereas the current velocity measurement techniques only yield point-wise data or data in a thin laser sheet measuring volume, in the best case. The most common current trajectory visualization technique, based on the use of tracer gases, only gives qualitative information. The objective of the TRAQ project is both the operational development of a new diagnosis tool that can be used outside laboratories to measure, in real-time, the extended three-dimensional trajectory and the velocity of particles immersed in fluids; and the application of the designed tool to scale 1 case studies. This tool, called 3DPTV (3D Particle Tracking Velocimetry), will initially be applied for Building science and technology and to bioreactor engineering, before being adapted to other industrial fields. 3DPTV is a quantitative measurement technique composed of particles of neutrally buoyant particles, also called tracers, a tracer illumination system, at least three time- synchronous cameras, and finally an algorithm which uses the recorded images to calculate the 3D trajectory of each individual tracer. Real-time access to 3D statistics involves developing and assembling, within the Institut Pascal, 3DPTV-specific smart cameras and the parallelization of existing algorithms. The project also aims to extend the lifetime of the tracers currently used in buildings by an interfacial chemistry study and to test the possibility of obtaining a thermochromic tracer. Measuring volumes will be expanded by devising a procedure allowing combining several different 3DPTV systems. Self-calibration of the camera network will also be investigated. Within TRAQ, 3DPTV will be applied to real cases such as studying the propagation of particles emitted during a coughing fit or the dispersion of pollutants in a dwelling. The data obtained will be made freely available to the CFD community. Those different technological barriers can be overcome thanks to the interdisciplinary research within the Pascal Institute, which harbors high-level researchers in electrical engineering, mechanical engineering, computer science and chemistry. The project is planned to last 48 months and should result in at least two patents, one on the new tracer and the other one on the real–time 3DPTV system. By addressing air pollutant monitoring, energy efficiency in buildings and the optimization of bioenergy production processes, the project must help to meet society's high expectations for indoor air quality and clean, efficient energy.