In Blackjack, deciding whether to hit, stand, double down, or split depends on the player's hand value and the dealer's upcard. Hitting is typically advised when your total is low and you need to improve your hand. Standing is often appropriate when you have a strong hand that you believe can win without risking additional cards. Doubling down is a strategic choice when you have a favorable hand, usually totaling 9, 10, or 11, and want to increase your potential payoff by doubling your bet after receiving one more card. Splitting can be beneficial when you have a pair, allowing you to play two separate hands, but should be approached with caution based on the specific pair and the dealer's card.