Principle purpose of this lecture is to present on Introduction to Cells. Cell, in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. Here briefly present on how to measure cell size; we can use microscopes to estimate the size of cells we are looking at; e.g. if we know the diameter of the field of our field of view we can then estimate the size of the cells.