Boisvert, V Armstrong, S Baines, J T M Bee, C P Biglietti, M Bogaerts, A Bosman, M Brandt, S Caron, B Casado, M P Cataldi, G Cavalli, D Cervetto, M Comune, G Corso-Radu, A Di Mattia, A Díaz-Gómez, M Dos Anjos, A Drohan, J Ellis, Nick Elsing, M Epp, B Etienne, F Falciano, S Farilla, A George, S Ghete, V M González, S Grothe, M Kaczmarska, A Karr, K M Khomich, A Konstantinidis, N P Krasny, W Li, W Lowe, A Luminari, L Ma, H Meessen, C Mello, A G Merino, G Morettini, P Moyse, E Nairz, A Negri, A Nikitin, N V Nisati, A Padilla, C Parodi, F Pérez-Réale, V Pinfold, J L Pinto, P Polesello, G Qian, Z Rajagopalan, S Resconi, S Rosati, S Scannicchio, D A Schiavi, C Schörner-Sadenius, T Segura, E De Seixas, J M Shears, T G Sivoklokov, S Yu Smizanska, M Soluk, R A Stanescu, C Tapprogge, Stefan Touchard, F Vercesi, V Watson, A Wengler, T Werner, P Wheeler, S Wickens, F J Wiedenmann, W Wielers, M Zobernig, G Region Selector Region-of-Interest 2003 2003 Among the many challenges presented by the future ATLAS detector at the LHC are the high data taking rate and volume and the derivation of a rapid trigger decis ion with limited resources. To address this challenge within the ATLAS second le vel trigger system, a Region-of-Interest mechanism has been adopted which dramat ically reduces the relevant fiducial volume necessary to be readout and processe d to small regions guided by the hardware-based first level trigger. Software ha s been developed to allow fast translation between arbitrary geometric regions a nd identifiers of small collections of the event data. This facilitates on-deman d data retrieval and collection building. The system is optimized to minimize th e amount of data transferred and unnecessary building of complex objects. Detail s of the design and implementation are presented along with preliminary performance results.