This book on TRIGONOMETRY has been specially written for the students of Science and Engineering. The basic purpose of the book is to provide the subject matter in a simple and lucid way so that the students could find no difficulty in understanding it. In terms of content, it covers the following chapters: Complex Numbers, Exponential Trigonometric and Hyperbolic Function of a Complex Variable (Separation into Real and Imaginary Parts), Logarithms of Complex Numbers, Inverse Circular and Hyperbolic Functions of Complex Numbers, Gregory Series, Trigonometrical Expansions and Summation of Trigonometrical Series. Each chapter of this book contains brief theory and fairly large number of examples and exercise. In selecting exercise variety has been the main consideration and it has been avoided to overload the book with too much drill work.