IT-Outsourcing is a concept that describes the supply of part or all of IT services by an external supplier company, known as IT-vendor, to a client company. IT-Outsourcing enables focusing on core business, cost reductions and increase of IT-resources efficiency. The purpose of this work is to discover, describe and analyze the most critical aspects in implementing IT-Outsourcing and the critical aspects of the ‘IS Improvement'' view of IT-Outsourcing for clients, to further let IT-vendors know how they can improve on delivering their services to their customers. The results are based on primary data, and they are also useful to the academic audience of IT-Management. This work leads to the conclusions on which the critical aspects in implementing IT-Outsourcing are and that the ‘IS-Improvement'' view of IT-Outsourcing for these four organizations is not valid, but the respondents'' indicated ways that IT-Outsourcing can be improved to achieve increased IT-resources efficiency.