Security is an increasingly important issue for networked services. However, since networked environments may exhibit varying networking behavior and contain heterogeneous devices with varying resources tunable security services are needed. A tunable security service is a service that provides different security configurations that are selected, and possibly altered, at run-time. In this paper, we propose a conceptual model for analysis and design of tunable security services. The proposed model can be used to describe and compare existing tunable security services and to identify missing requirements. Five previously proposed services are analyzed in detail in the paper. The analysis illustrates the powerfulness of the model, and highlights some key aspects in the design of tunable security services. Based on the conceptual model, we also present a high-level design methodology that can be used to identify the most appropriate security configurations for a particular scenario.