With the recent conflict in Bosnia-Herzegovina being in the world news front, the Adriatic Sea has become an important strategic operating area for the North Atlantic Treaty Organization (NATO) and for the U.S. Navy. The NATO Undersea Research Centre located in La Spezia, Italy carried out the Otranto Gap (OGAP) project in 1994 and 1995 to assess the oceanography and bottom geology of the Southern Adriatic. As part of this project, the OGEX1 cruise was conducted between 19 and 24 May 1995 with focus in the Otranto Strait, through which the Adriatic is connected to the rest of the Mediterranean basin, and on the Albanian shelf. In this thesis the water masses present in the southern Adriatic are studied and the P-vector method is used to estimate the absolute geostrophic circulation, based on the hydrographic data (CTD, XCTD and XBT) collected during the OGEX1 cruise. The P-vector results are interpreted and compared with other oceanographic data sets acquired during the OGAP project, namely, current meter and ADCP data, drifter tracks and thermal satellite images. The absolute geostrophic velocity at 40 m, derived by the P-vector method, shows rather well the expected cyclonic circulation in the Southern Adriatic north of 41 deg N. In contrast, the results in the Otranto Strait area need to be interpreted with caution. Current meter data show that this area is very ageostrophic. A comparison between geostrophic and directly measured vertical velocity shears indicates a large departure from geostrophy in this area. The wind is shown to be a main factor forcing the circulation in the Adriatic,either directly or through changes in sea level.