A button in your email has a very specific goal: to get your reader to complete an action. What is an email good for if it doesn't call someone to act? For that reason, it could be argued to be the most important element of your entire email. You've spent hours stressing about copy and images and subject lines and send times and segments, but all of those things lead to this little boxy button. What color should it be? Where should it be placed? How big should it be? What words should you use? How many should you have?