Theta decay, also known as time decay, is the rate at which an options value decreases as time passes and the option approaches its expiration date. It is one of the few consistencies that option traders can rely on, as the passage of time is certain and most options will continue to lose value over time. Theta is expressed as a negative number for option owners, meaning that the extrinsic value of the option decreases as time passes, all else being equal. The rate of decay tends to accelerate as the option approaches its expiration date, which is why it is sometimes referred to as "time decay". Theta varies depending on a contracts strike price relative to the underlying assets price, with the fastest decay occurring for contracts at-the-money. Theta is one of the main Greeks that option buyers should worry about since time works against long option holders. On the other hand, theta is positive for short options, indicating that time decay is theoretically beneficial for the option seller.