Requirements Engineering (RE) helps to identify, analyze and document system requirements. Proper analysis and understanding of system requirements is important because it helps to discover any requirements defects or mistakes in the early stages of development. Several processes and techniques have been developed to assist requirements engineering activities for product development. However, most of the existing product line practices do not comprise all the RE activities required for proper identification, analysis and understanding of product line requirements. In this paper, we propose a systematic and iterative RE approach for product line development. The approach includes all the activities required for proper identification, analysis, modeling and specification of product line requirements. In addition to this, it proposes several specific techniques such as aspect-orientation or separation of concerns, product maps and extensible markup language (XML) to assist different RE activities. The concept of aspect-oriented programming is used for analyzing the common and variable requirements. Product maps are used for determining the scope and characteristics of the product family. Extensible markup language (XML) is used for requirements specification and traceability.