Price and output determination under perfect competition and imperfect competition during a short period and long period reveal that during the short period under perfect competition, profit, loss and normal profit situation are found as firms have no say in price determination as it is dictated by the industry and an individual firm is a price taker and takes a decision regarding the quantity of output. During long period firms earn a normal profit under perfect competition as there is free entry and exit of firms in the industry. All the characteristics of perfect competition are imaginary and unrealistic.