Methods for analysing interlanguage have long aimed to capture learner language in its own right. By surveying the cognitive methods of Error Analysis, Obligatory Occasion Analysis and Frequency Analysis, this paper traces reformulations to attain this goal. The paper then focuses on Emergence Analysis, which fine-tunes learner language analysis by measuring the "onset" of spoken grammar as hypothesised in Processability Theory (PT) (Pienemann, 1998, 2005a). Since doubts have been expressed regarding the emergence approach's rigour and pedagogical relevance, a study is presented which aims to provide a more in-depth and wide-ranging account of the "onset" of English grammar. Having sketched and amplified Processability Theory's predictions, the paper applies emergence analysis to the longitudinal development of two adolescent ESL learners. As well as exemplifying the rigour of the emergence procedures, the results show overall support for the more comprehensive predictions. The paper concludes that learner language analysis does not have a deficit emphasis on transition to the target language, as claimed by Firth and Wagner (1997, 2007). Indeed, such methods have a role in assessing developmental readiness in a learner-oriented approach to grammar teaching. (Contains 1 figure, 7 tables, and 1 endnote.)