The ability to attend the intended event or stimulus while ignoring other information is one of the primary construct researched in the cognitive research domain. Most of the studies in the area of sustained attention or vigilance are broadly categorized into two categories (i) Researches related to the exploration of the vigilance decrement phenomenon and factors influencing it. (ii) Studies attempting to find out different variables which might improve the vigilance performance. In spatial cueing visual vigilance task, a valid cue is a condition in which target or signal is present at the exact location of cue, whereas during invalid cue condition target is presented in any of the place except the cue's location. This book is an empirical work to study how an observer's performance gets affected by the increase of task load during varied cue-target onset asynchrony (CTOA).