Although some recipients benefit from exposure to sophisticated and sensitive support messages, the effects of these messages are moderated (sometimes substantially) by characteristics of the recipient, the helper, and the situation. Thus, enhancing the success of helpers who provide support requires a comprehensive explanation of why support messages are effective in some circumstances but less effective in others. To understand why supportive messages work, we must understand how these messages are worked on (i.e., processed) by their recipients. This chapter uses a recently developed dualprocess theory of supportive message outcomes to explain how and why multiple variables moderate the effects of supportive messages. We provide a comprehensive review of published research findings concerning the demographic, personality, cognitive, and situational moderators of supportive messages and show that these moderators can be interpreted as (1) factors influencing the message recipient’s ability and/or motivation to systematically process these messages or (2) environmental cues that quickly trigger responses to the message.