Recent work on ellipsis has promoted the hypothesis that syntax plays a limited role in the computation of the required isomorphism conditions between elliptical clauses and their antecedents. For example, Merchant (2008, An asymmetry in voice mismatches in VP-ellipsis and pseudogapping. Linguistic Inquiry 39. 169–179; 2010, Vocal and argumentative oppositions. Paper presented at the Workshop on Morphological Voice and Its Grammatical Interfaces: Theoretical Modeling and Psycholinguistic Validation. University of Vienna, June 25–27; 2013, Voice and ellipsis. Linguistic Inquiry 44(1). 77–108) shows the relevance of the syntactic heads related to voice and argument structure. Focusing on sluicing, Chung (2013, Syntactic identity in sluicing: How much, and why. Linguistic Inquiry 44(1). 1–44) adds Case-valuing heads to the list and Takita (2013, Strengthening the role of case in ellipsis. Paper presented at the Identity in Ellipsis Conference, Leiden University, September 20–21) argues that Case is the only necessary syntactic information. This article focuses on sluicing and provides further support for the role of syntax in ellipsis isomorphism conditions. However, it shows that Case cannot be a distinct primitive component in isomorphism computation since grammatical sentences will be wrongly predicted to be ungrammatical and vice versa. The analysis of cross-linguistic data provides evidence that a syntactic requirement, labeled the Remnant Condition, is indeed necessary. This condition is based on the general notion of unrecoverable information and on the proposed concept of Id(entity)-source, a restricted version of the sluiced clause, the derivation of which has to comply with all the required (syntactic and non-syntactic) identity conditions and, crucially, must be sensitive to the morphosyntactic information present both in the antecedent clause and in the remnant. The goal of the Remnant Condition is to ensure the correct syntactic integration of the remnant phrase in the Id-source. Sluicing facts related to Case, voice and argument structure, merging and sprouting are captured by a combination of the Remnant Condition, general principles of grammar and non-sluicing-related syntactic principles internal to the different languages.