The presented thesis is divided into two distinct parts. The subject of the first part is the ATLAS high-level trigger (HLT), in particular the development of the HLT Steering, and the trigger user-interface. The second part presents a study of inclusive supersymmetry searches, including a novel background estimation method for the relevant Standard Model (SM) processes. The trigger system of the ATLAS experiment at the Large Hadron Collider (LHC) performs the on-line physics selection in three stages: level-1 (LVL1), level-2 (LVL2), and the event filter (EF). LVL2 and EF together form the HLT. The HLT receives events containing detector data from high-energy proton (or heavy ion) collisions, which pass the LVL1 selection at a maximum rate of 75 kHz. It must reduce this rate to ~200 Hz, while retaining the most interesting physics. The HLT is a software trigger and runs on a large computing farm. At the heart of the HLT is the Steering software. The HLT Steering must reach a decision whether or not to accept an event within a time budget of less than 40 ms (LVL2) and under 4 s (EF), averaged over all events. Its decision is based on the results of customised trigger algorithms. These algorithms perform a specific data reconstruction task in a small geometrical detector region. The Steering controls the flow and execution of these algorithms, such that only interesting (typically high-pT) detector regions are exami ned in several steps. The principles and design of this HLT selection strategy are discussed in detail. The final implementation of the Steering, including all required features such as pre-scaling, is presented. Furthermore, commissioning results from technical and cosmic ray runs are described. Finally, the common interface to all trigger results for off-line analysis and debugging is presented. Supersymmetry (SUSY) is a theoretically attractive scenario for physics beyond the SM which may also provide a suitable dark matter candidate in models where R-parity (+1 for SM and -1 for SUSY particles) is conserved. If SUSY is realised at a mass scale of ~1 TeV, as favoured by several arguments, it will be accessible at the LHC at CERN within the first years of data-taking. The second part of this thesis presents the inclusive one-lepton search strategy for generic SUSY models with R-parity conservation with the ATLAS detector at the LHC. The signature of this search mode is based on large missing transverse momentum from undetected neutralinos, multiple high-pT jets and one lepton. The corresponding discovery reach is shown for the first fb^-1 of ATLAS data. Furthermore, background estimation techniques for inclusive SUSY searches are discussed. A novel background determination approach, denoted the Tiles Method, is proposed, which translates prior knowledge on the SM distributions of discriminating variables in a two or hi gher dimensional grid into an estimate of the abundances of SM and beyond-SM events. Depending on the grid granularity, the abundances are calculated by solving a system of linear equations or by minimising a log-likelihood function. The method does not rely on assumptions on background dominance for particular values of the variables, nor does it require iterations. Systematic effects due to uncertainties in the simulated prior distributions are investigated. Results for various mSUGRA scenarios are presented
