An alternation between clauses is treated as a diathetical alternation (a) if one or more semantic roles associated with the main verb exhibit differential grammatical (i.e., morphological or syntactic) encoding, (b) if the overt lexical expressions have same lexical roots, and (c) if the clauses approximately share at least the meaning and truth conditions of the semantically less specific clause alternant. This qualifies as diathesis what has come to be known as the canonical passive, impersonal passive, non-canonical passive, pseudo-passive, anticausative, the dative alternation, and the locative alternation, among others. The focus of this article is on the semantic restrictions governing a clause’s participation in various diathetical alternations across the modern Germanic (standard) languages. Semantic differences between alternating clauses are captured using a sophisticated semantic role account. Grammatical encoding of diathesis is described in a theory-neutral manner using the four-case system of the old Germanic languages as a tertium comparationis and syntactic function notions from descriptive typology. Diatheses are differentiated by the semantic roles that are fore- and backgrounded by means of the syntactic functions they bear. The roles that alternate in grammatical coding are foregrounded in the clause in which they have the higher syntactic function in a syntactic function hierarchy, and they are backgrounded in the clause in which they have the lower syntactic function. In a first set of diatheses, alternations are described in which the proto-agent role is backgrounded and a proto-patient is foregrounded. This set includes a “patient passive” and the “anticausative domain.” In a second set of diatheses, the proto-agent is again backgrounded, but now the proto-recipient is foregrounded. This is illustrated using the “eventive recipient passive.” Completing this pattern, the “locational passive” represents a diathetical pattern in which the proto-agent role is backgrounded once more and the proto-locational role is foregrounded. Other types of diatheses in which the proto-locational is foregrounded and the proto-patient is backgrounded are exemplified by means of the location/possession alternation (dative alternation) and the location/affection alternation (e.g., locative and applicative alternations).