A patient-like participant of a transitive verb (e.g. the O-argument of ‘hit’) or intransitive verb (the S-argument of ‘die’) is generally indexed by means of Undergoer alternations in the verb stem, as described in this chapter. In example (384), the form ɣamuk is the 2nd person singular Undergoer form of the verb wamuk ‘hit’. (384) no- ɣ-amuk 1.A- 2SG.U-hit ‘I hit you.’ The general features of Undergoer indexing are covered in §10.1, including indexing of transitive O-arguments and patientive S-arguments (§10.1.1). An important quirk is that many common agentive intransitive verbs, such as motion verbs, exhibit a ‘reflexive’ indexing pattern, in which the agent is indexed simultaneously by the Actor prefix and Undergoer alternations - the so-called middle indexing pattern (§10.1.2). The term alternation is used instead of ‘affix’, because the the exact locus of exponence (as a prefix, suffix or infix) is determined by the inflection class of the verb lexeme, and for some verb stems no segmentation is possible. The formal characteristics of verb stems are introduced in §10.2. A crucial observation is that the presence of Undergoer indexing is lexically restricted to about half of the verbal lexicon. Verb lexemes that participate in Undergoer indexing are alternating, and verb lexemes that do not are invariant (§10.2.1). The four inflectional classes, distinguishing prefixing, infixing, suffixing and double-marking verb lexemes, are described in §10.3. Alternating verbs that only select for inanimate S/O-arguments are treated in §10.4. These lack the full range of person/number stems of other verbs, and only reflect the gender of the Undergoer argument. Finally, §10.5 describes verb suppletion according to the number of the S/Oargument, which affects more than a dozen verb lexemes.