We are all familiar with mobile phones and it has become an integral part of our life. What is the working principle or what is the major control element of a mobile phone? From a simple watch circuit to a high frame computer, it uses the same principle or idea of construction. They are all powered by a microprocessor or a controller. Microprocessor and controller have become an integral part of almost all the electronic goods that we are using now because of its easy implementation. In the past after the invention of transistor if we have to design a circuit for solving a problem like measuring temperature it was very tedious. This was because implementing this logic requires some difficult calculations and again converting this logic into circuit is harder than the logic derivation. Let the circuit be implemented after this entire hard works, then what will be the situation, the size of the circuit will be of size of a large room. To reduce the size integrated circuits were began to use. This helped to reduce the size to an extent. But the difficulty in designing circuit and implementing it remained the same. This time in 1930 or 40s it became essential to develop a control circuit for the space exploration of Apollo and it has led to the invention of microprocessors. The circuit designed for the Apollo was similar to a computer which contains a main control unit, memory, power supply unit etc. Later microprocessors were replaced by microcontrollers. A unit consisting of a controller and the associated parts for its working can be called embedded system. Embedded systems have become popular after its invention and have changed the electronics field a lot. A circuit which is designed to do a specific task or for a series of tasks can be called embedded systems. In modern electronic industry it is the factor behind every electronic product. A microprocessor or controller can be programmed to implement a function. Thus the tedious task of circuit designing has paved way to programming. This helped to bring down the effort a lot. This is the key factor that has replaced the ordinary transistor and IC circuits. As Early said embedded system is consisting of a processor or a controller. There will be some associated components required for the working of controller. The main part of the circuit will be the controller or processor. The controller architecture consists of processing unit, memory unit, registers, input/output ports, timer or counter. These parts are integrated in the single chip. Processing unit will be carrying out mathematical and logical calculations. The results of these calculations were stored in the registers. Input and output unit helps to exchange data between the controller and the user. The whole activities of the controller are synchronized by the timer frequency. The normal frequency of controller is around 20 MHz With this interval of milli second is required for the completion of a process. One of the advantages of microcontroller based system is that it can integrate both mechanical and moving parts in the circuit. Examples of circuits which uses controller are digital watches, MP3 players, mobiles, computers, digital cameras, DVD players, washing machines and even microwave ovens. Also some industrial machines dedicated for a particular task can also be designed using microcontrollers. Microcontrollers can be programmed in high level and low level. High level language can be written with the help of languages like C, C++ etc. But low level programming is a bit difficult task. It is written in machine language. So in order to write low level language one should study the pneumonic of the microcontroller instructions. These written programs determine the functions associated with a controller. Larger the functions to implement larger will be the program. The assembly program of the controller is stored within the Rom of the controller. When microcontroller is powered the program in the Rom is loaded and functions can be implemented. Data which is to be processed is stored on the Ram and registers. Manipulations can be done on this data and results can be obtained from the processing unit. Data present in Ram will be available only if there is power. So for future use data present in Ram is to be moved to secondary storage devices like hard disk or CD. Microcontrollers not only helped in reducing the task for designing a circuit but also it helped to reduce the size of devices like mobile phones and MP3 players. In future we can hope that microcontrollers will rule the electronic industry.