From the beginning the order took on both the work of popular preaching and the work of studying and teaching philosophy and theology, so as to harness the new intellectual developments of the time in the service of the gospel.

At the date St. Dominic established the Order, new orders had to adopt existing Rules, and St. Dominic adopted the Rule of St. Augustine, to which he had been used while a Canon at Osma in Spain. It is so basic and simple a Rule about living together in peace and mutual respect in a religious community dedicated to a committed Christian life and service, that the orders which adopted it could supplement and adapt it with their own constitutions.

That fact allowed St. Dominic to give the Order a set of Constitutions strikingly different from those of other orders then recently founded. The Constitutions were thoroughly revised in the period 1960-68, but preserving the emphases of St. Dominic as they need to appear today, when the Order is much smaller in many areas than it was in the Middle Ages.

In large Mediaeval priories it was possible for many of the brethren to be excused some of the monastic observances for the sake of their work, while the rest kept the whole regime going; nowadays, with smaller communities, we have to limit the number of Offices and meals celebrated in common, but no one is automatically excused attendance because of his job.

Since the Order was founded for the good of souls, St. Dominic allowed superiors to dispense members from the observances when they would impede preaching and study. He avoided having numerous detailed and fussy rules in the Constitutions. Rather, to promote personal responsibility and local initiatives to meet local needs, he made the Order democratic, with superiors elected and sharing their authority with chapters and councils, and had it stated in the Constitutions that disobeying the Constitutions did not make one automatically guilty of a sin of disobedience.

From the beginning the Order has included not only communities of friars but monasteries of enclosed nuns who contemplate the Word preached by the brethren and pray for the apostolic fruitfulness of their activities. Shortly after, it acquired chapters of lay members living in the world but committed to a life of penance (asceticism), prayer and Christian good works. In the modern period, these were joined by sisterhoods living the 'mixed' life, combining community life with such active tasks as education and nursing. The first three groups constitute the Order proper; the last is associated with them in the wider 'Dominican family'.