This book is part of a series designed to provide engineering students in colleges and universities with a mathematical toolkit, each book including the mathematics in an engineering context. Worked examples and problems with answers are included. It is aimed primarily at HNC/HND students and first-year undergraduates. Thus the mathematics assumed is that in BTEC National Certificates and Diplomas or in A level. The pace of development has been aimed at the notional reader for whom mathematics is not their prime interest or "best subject" but who need the mathematics in their other studies. This book is concerned with complex numbers and their application in, primarily, electrical/electronic engineering. A familiarity with basic algebra is assumed. The aim has been to include sufficient worked examples and problems to enable the reader to acquire some understanding and proficiency in the handling of complex numbers and, in particular, their use to describe phasors in electrical circuit analysis.