A triangle is a geometric figure with three sides and three angles. When two rays meet to form an angle, the common endpoint is called the vertex of the angle. A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices A, B, and C is denoted. In Euclidean geometry, any three points, when non-collinear, determine a unique triangle and simultaneously, a unique plane. The three sides are connected end to end at a point, which forms the angles of the triangle. The sum of all three angles of the triangle is equal to 180 degrees. Other concepts covered in this chapter include Similar Polygons, Equiangular Triangles, and Similar Triangles. This chapter also discusses various theorems such as the Thales Theorem, Converse of Thales Theorem, Midpoint Theorem, Angle-Bisector Theorem, AAA-similarity, SSS-similarity, SAS- similarity, Pythagoras Theorem and more.