Trigonometry is an important field of mathematics. It deals with the study of relationship between angles and lengths of triangles. Such studies have led to the development of trigonometric functions. A trigonometric function relates the angles of a triangle to the lengths of the sides. Sine, cosine and tangents are the most prominent trigonometric functions. Such functions are useful in the study of triangles and for modeling periodic phenomena. The study of planar right-angle triangles is generally dealt in trigonometry. These triangles are two-dimensional triangles, which have one angle equal to 90 degrees. Spherical trigonometry studies the triangles on spheres. Trigonometry and trigonometric functions are applied to a wide array of fields such as optics, astronomy, medical imaging, cartography, economies, seismology, etc. This book provides comprehensive insights into the field of trigonometry. Such selected concepts that redefine this field have been presented herein. Coherent flow of topics, student-friendly language and extensive use of examples make this book an invaluable source of knowledge.