Prime objective of this lecture is to present on what is Well Logging. Well logging chronicles the depths, subsurface formations and events encountered while drilling. Well logs can include visual observations or be made by instruments lowered into the well during drilling. Well log is a continuous record of measurement made in bore hole respond to variation in some physical properties of rocks through which the bore hole is drilled. Traditionally Logs are display on girded papers shown in figure. Now a days the log may be taken as films, images, and in digital format.