Cloud services are emerging as convenient and attractive for the industrial control system community. There are however a need for security related considerations regarding what parts of a system that can be transferred to the cloud. This report presents a categorization of cloud services, their service levels and related security challenges. With that in mind, a number of security related advices aimed at system owners and operators of industrial control systems are formulated. The conclusions of the study stress the importance of having access to IT security competence in-house, as well as resources to cope with IT security related incidents when transferring parts of industrial control systems to the cloud.