Framework for conditionally attaching web service policies to a policy subject (e.g., a web service client or service endpoint) at subject runtime. In one set of embodiments, a constraint expression can be defined that specifies one or more runtime conditions under which a policy should be attached to a policy subject. The constraint expression can be associated with the policy and the policy subject via policy attachment metadata. The constraint expression can then be evaluated at runtime of the policy subject to determine whether attachment of the policy to the policy subject should occur. If the evaluation indicates that the policy should be attached, the attached policy can be processed at the policy subject (e.g., enforced or advertised) as appropriate. Using these techniques, the policy subject can be configured to dynamically exhibit different behaviors based on its runtime context.