The short answer is yes; you have to send a thank you note!

The longer answer... Generally, if you opened the gift in person and made a thank you in person, you don't need to send a thank you note. However, thank you notes are critical if you did not open the gift, or if it came by mail because the giver wants to know that you got the gift and knew it was from them! Most people try to pick a gift that means something and is an expression of their love and esteem for you and would like to know they did a good job. I know some gifts really miss the mark, but the giver did go shopping with you in mind and that is worth something. Letting them know you appreciate them is a small price to pay. Thank you note should be on paper, though a quick email to say the note is on its way is allowed! Outside the gift giving realm, you should also thank a hostess who threw a party in your honor, like a shower, anyone who's home you stayed in overnight, or anyone that sent cards or flowers while you were ill, or to a loved one's funeral. There is a more grey area with closer friends and relatives that help with a favor or put you up for a few nights, or with job interviews where you don't really have to send a note, but it couldn't really hurt! People like to be appreciated and a nice thank you note will almost ensure another nice present in the future! You can skip the notes for dinners or gifts that are part of a business pitch as you've provided your time and attention to the presentation and the gift was not personal, but part of a business venture.