This book presents the outcomes of a research project that sought to investigate the interplay between L1 and L2 in the investigation and analysis of Arabic-speaking EFL learners' errors while using definite and indefinite articles in English. The study had two primary goals: First, to identify errors in the use of articles and classify them into three categories, namely addition, omission, and substitution. Second, to attribute these errors to their possible source, namely interlingual or intralingual. This was an attempt to postulate whether L1 Arabic or L2 English played a key role in EFL learners' erroneous use of articles. The study participants were 60 male Saudi EFL learners, whose ages were between 18 to 22 years, from a preparatory year EFL program in a large-sized university in Saudi Arabia. They were mainly from two streams, namely Science and Humanities, with different English proficiency levels. In addition, 10 EFL teachers of different nationalities and varied teaching qualifications were interviewed so as to explore how they perceived such learners' errors is the use of articles and what ideal solutions they adopted to aid learning.