Trust and trustworthiness are two notions that have been discussed extensively in the computer science community, e.g. trust in online banking services.We argue for a broad view on trust, namely trustworthy behavior of online services. We propose solutions enabling online service developers to reason about, and deal with issues of trustworthy online services, from concerns to actual implementations, and assessments. The view on trust in this thesis involves viewpoints on what stakeholders can have trust in, and the need to exhibit and suggest trustworthiness in online services. Trustworthiness and other relevant theories are also discussed. Three main results supporting design and maintenance of trustworthy online services will be introduced. First, a trust framework in the context of online services is introduced, specifying a number of concepts that enhances and clarifies how trust can be addressed. The framework enables an informed analysis, implementation, and assessment of solutions to trust issues based on identified trust concerns. Secondly we present how the concepts of the framework can be interconnected. The concepts enables us to reason about stakeholders trust concerns in relation to deployable solutions called trust mechanisms that are implemented in order to exhibit proper signs suggesting trustworthiness. These signs, we argue, serve as input for stakeholders trust assessment. The interconnected framework opens up for a discussion on how deployed solutions in an online service correspond to certain stakeholders trust concerns. Finally a tool for online service designers is presented, the trust management life cycle. This is an approach enabling an informed design practice that emphasizes on a trustworthy design of online services. The use of the cycle is illustrated by the use of a deployed online service.