Spelling is a foundational literacy skill that supports both word reading and written expression. For students with or at risk of a learning disability (LD), difficulties in spelling often constrain the fluency and complexity of writing, making effective interventions essential. Yet, the conclusions drawn about intervention efficacy depend heavily on how outcomes are measured. This review synthesizes outcome measurement practices across 59 spelling intervention studies conducted over the past five decades. All outcome measures ( n = 233) were coded by type (researcher-developed vs. norm-referenced) and by linguistic level (sublexical, lexical, sentence, discourse) using the Interactive Dynamic Literacy (IDL) framework. Descriptive analyses revealed that nearly four out of five outcomes were lexical, most often researcher-developed lexical-level spelling probes, with comparatively few outcomes at the sentence or discourse levels. Standardized assessments were similarly concentrated at the word level, with the Wide Range Achievement Test–Spelling subtest and Test of Written Spelling most commonly used. Finally, the pairing of proximal and standardized outcomes was inconsistent, particularly among group designs. Taken together, findings highlight a measurement bottleneck: spelling interventions are evaluated primarily through lexical-level accuracy, offering limited insight into whether gains transfer to the higher-level writing processes for students with or at risk for LD.