The conceptual background of the applied behavior analysis is presented with the emphasis on the operant behavior paradigm. The applied behavior analysis as a behavior science is characterized ⑴ by its interests in human behaviors significant for the individuals as well as for the society, ⑵ by its attempt at an expansion of laboratory behavior principles to real and practical human behaviors, ⑶ by its emphasis on the intensive observation and analysis of single subject behavior to examine the effects of experimental variables and to discover functional relations, ⑷ by its detailed description of procedural control to generate engineering possibility for the control of hehavior, and ⑸ by its interests in collating and conceptualizing the related studies to supplement, expand, and revise the existing system of applied behavior analysis. The foregoing conceptual discussion is followed by a set of principles typically used in the behavioral clinic. Four general steps are identified to subsume ⑴ the definition of behavior as practicable and precise as possible, ⑵ the analysis of baseline prior to the initiation of modification of behavior underquestion (often using the time sampling method), ⑶ the behavior control by the use of shaping procedures involving the successive approximation and differential reinforcement and finally ⑷ the generalization and stabilization of behavior acquired under the limited controlling environments. The reversal technique and the multiple baseline technique are introduced and briefly examined to aid in testing the reliability of control of experimental variables. It is argued that the time is ripen to consider practicality, rather than verbose theories, in such practical behavior control as classroom, behavior clinic, and psychotherapy.