Over the past 5 years the concept of large groups of grower-finisher pigs has expanded. Not only does it include extensive (outdoor raised) operations and hoop structure barns but the conversion or special construction of conventional housing modified to accommodate groups much larger than the traditional 20 pigs per pen. Why? There were four drivers: A desire to reduce construction costs by simplifying penning and barn design; herd sizes were becoming large enough that large groups could be formed without commingling ages; electronic sorting technology provided a means to deal with the critical task of accurately separating at market without high labour costs; and finally improved use of space could improve profitability and relieve the crowding effect brought on by higher sow productivity over the past decade.