The work reported in this book is based on empirical data collected from three groups of speakers: native speakers of English and Persian and Persian speakers of English as an L2. The study addresses, among others, two of the most important aspects of L2 speech: 1) it provides an extensive sociolinguistic description of L2 by conducting multivariate analyses on features of English spoken by Persians to throw light on the issue of the systematicity of interlanguage, 2) it explores the relationship between native language of the speakers and their L2, on one hand, and the relation between the L2 and the language L2 learners try to acquire, on the other. In other words, it examines the sources of the patterns of variability found in second language speech. The focus of the investigation is the variable contraction (deletion) of auxiliaries and variable use of relative and resumptive pronouns by low and high proficient L2 learners. This book, therefore, provides ‘hard evidence'' for the systematicity of L2 speech at different proficiency levels and can be used by students and scholars interested in the sources of variation/patterns in the so-called interlanguage of L2 learners.