Even a few years back, the main concern of consumers of electricity was the reliability of supply. Here we define reliability as the continuity of electric supply. Even though the power generation in most advanced countries is fairly reliable, the distribution is not always so. The transmission systems compound the problem further as they are exposed to the vagaries of Mother Nature. It is however not only reliability that the consumers want these days, quality too is very important to them. For example, a consumer that is connected to the same bus that supplies a large motor load may have to face a severe dip in his supply voltage every time the motor load is switched on. In some extreme cases, he may have to bear with blackouts. This may be quite unacceptable to most customers. There are also very sensitive loads such as hospitals (life support, operation theatre, patient database system), processing plants (semiconductor, food, rayon and fabrics), air traffic control, financial institutions and numerous other data processing and service providers that require clean and uninterrupted power. In several processes such as semiconductor manufacturing or food processing plants, a batch of product can be ruined by a voltage dip of very short duration. Such customers are very wary of such dips since each such interruption cost them a substantial amount of money. Even short dips are sufficient to cause contactors on motor drives to drop out. Stoppage in a portion of a process can destroy the conditions for quality control of the product and require restarting of production. Thus in this changed scenario in which the customers increasingly demand quality power, the term power quality (PQ) attains increased significance. Transmission lines are exposed to the forces of nature. Furthermore, each transmission line has its loadability limit that is often determined by either stability considerations or by thermal limits. Even though the power quality problem is a distribution side problem, transmission lines often have an impact on the quality of power supplied. It is however to be noted that while most problems associated with transmission systems arise due to the forces of nature or due to the interconnection of power systems, individual customers are responsible for a more substantial fraction of the problems of power distribution systems.