Whether trying to win Presidential primaries, trading stocks, or playing sports, performance-enhancing effects of psychological momentum (PM) are widely accepted. But, does initial success (S1) lead to subsequent success (S2) in and of itself due to increased know-how on one's and opponents’ performance or because it creates psychological force (momentum) that mediates this relationship? We review research on the phenomenon and show its strong empirical foundations in various domains of human performance. To advance research, we present an organizing theoretical framework that proposes both mediating and moderating effects of PM as mechanisms to explain why success breeds success in general. Initial success is critical for PM and has 3 types of effects: intensity, frequency, and duration. Whether performing alone (trader) or against an opponent (tennis player), perceptions of self as a performer (Sp) and of opponent as a performer (Op) are at the center of PM. The theory posits that the more the initial success separates the two (Op/Sp), the greater the PM. These and associated perceptions, however, have to turn into an increased subjective probability of winning or succeeding before PM becomes a psychological force. Evidence supports the mediating mechanism since initial success increases PM, which in turn enhances subsequent success. When initial success with PM leads to a greater likelihood of subsequent success than without PM, PM then modifies (“moderates”) the S1–S2 relationship without PM's independent effect on S2. There is also tentative evidence for a moderated mediation effect as the influence of PM seems to be greater for male than female performers. Areas of future research are highlighted.