This paper covers the topic of randomized controlled trials in social, educational, criminological, health, and other human service sectors. It is studded with illustrations from developed and developing countries. We address the basic ideas that underlie trials in different ways, and cover contemporary definitions and vernacular, some history, and idea of cumulating evidence from such trials including recent work on replication and meta-analyses. Standards for evidence and reporting are considered. We attend to statistical matters and also recognize important non-statistical matters that must be taken into account in designing and executing such trials. Cluster randomized, place randomized, and other designs for such trials are covered on account of their increasing importance.