The phrase 'group decision-making' may sound very formal and important, but group decision-making is really about people making arrangements to do things with other people. Two friends who meet on the street and decide to go for a pint later in the evening; a married couple choosing wallpaper for their living room; or a small football team making a decision not to participate in a regional tournament. Group decision-making is something that we all do every day - we use language to make decisions with other people. But what do we actually do when we make decisions in groups? And what is a group decision? This doctoral dissertation investigates these questions using linguistic methods, focusing on interaction and language during group decision-making. Recordings of many different kinds of groups - from city district committees to married couples - are analyzed in detail. The dissertation also contains a detailed investigation of the concept of decision, a model for analyzing arguing in group decision-making, and a quantitative (corpus linguistic) study of the language of group decision-making.