When evaluating a position, besides determining the balance of territories, an important consideration is the identification of weak groups. Positions often arise in professional games where one side seems to have secured a sizable amount of iron-clad territory, while the other side has little or even no area of the board that he can count on as territory. However, if the side with all the territory has a weak group, the other side can rectify this territorial imbalance by attacking that group. The purpose of the attack is not to capture the weak group, but to harass it and, in the process, build influence that will negatively affect the opponent's groups elsewhere on the board. Even when the territorial balance is relatively even, one side can gain an advantage by attacking a weak group. On the other hand, failure to reinforce a weak group can result in the disruption of the territorial balance. This book covers all the techniques of attacking and defending weak groups. Each of the nine chapters starts with a few examples of the technique under study, then continues with a few problems showing how that particular technique was used in a professional game. The tenth chapter presents additional problems whose solutions draw upon the techniques studied in the preceding nine chapters.