Aim of this master thesis was to investigate Leak-Off Tests in more detail. As throughout the industry different explanations for the actual shape of the pressure vs. time and pressure vs. volume curve exist, the part where the first deviation from a straight line is observed should be investigated in more detail. A literature research has been conducted, studying professional literature published throughout the industry as well as publications of different universities and scientific literature. First, all factors affecting a Leak-Off Test as well as the Leak-Off Test procedure had been reviewed in detail. Data available at OMV was studied and examples being calculated. Aim was to identify the impact of the single effects on such a test. Focus of investigation was the linear region in the first place. Reason was to narrow down the possible effects responsible for the first deviation. These effects, termed cased hole effects, include casing expansion and mud compressibility as well as gas trapped in the drilling fluid. Furthermore, operational influences like the way of measurement are discussed, to finally closer investigated effects that might be a possible reason for the observed behavior. These effects, termed open-hole effects, are such as fracturing, filtration but also preexisting fractures. Moreover, theories explaining what happens at the Leak-Off Point are discussed and evaluated. Leak-Off Test data is analyzed in more detail and conclusions on permanent formation damage and what causes the slope of the pressure vs. volume chart to decline after the Leak-Off Point has been exceeded and therefore what the physical meaning of this point are made.