The changes in total output on account of the change in the factors of production in the same proportion is called the returns to scale. In the long run all the factors of production are variable and even the scale of production can be changed according to the demand for various goods and services in the economy. Returns to scale is concerned with long term production function. It is studied with the help of iso-product curves. Returns to scale are of three types (1) Increasing returns to scale, (2) Constant Returns to Scale, (3) Diminishing Returns to Scale.