A Product Line is a set of products with common elements and variable features. Including Product Lines in an overall development strategy tailored to the commercial and/or industrial context delivers significant benefits: products that are more suitable, reduction in cost, shorter development timescales, quality improvement, etc.This work, Systems Product Line Engineering, brings together a summary of the state-of-the-art with lessons learnt from industrial experience in implementing Product Lines of various kinds, in terms of marketplace, number of applications, degree of variability, etc. It is resolutely practical, and is intended to complement existing Systems Engineering manuals; indeed, it adopts the same process structures.