In the last couple of years several wells were drilled and completed in Northern Germany using 4-1/8” slim-hole drilling technology. Most of these wells were sidetracked conventionally and others were initially planned as a slim-hole design. The use of smaller diameters allows reduced costs in tubular, a smaller rig size and saves time. Serious problems developed while drilling the 4-1/8” section through the Stassfurt Carbonate and Rotliegend formations with several different downhole motor BHAs. The issues involved directional drilling (steering) problems, high lateral vibrations and reduced mechanical reliability of slim diameter tools. Problems resulted in downhole tool failures and in several roundtrips. Within this thesis five wells were reviewed and the problems were analyzed to make recommendations for improving the drilling performance of future 4-1/8” sections. The thesis also outlines future technology developments to make slim-hole drilling a more effective technique.