This paper represents a new approach to size effects in timber. The design process in Canadian Standards Association code CAN3-086-M80 allows for size effects in shear and in tension but not in bending. A large number of bending test results arc analyzed in this paper to show that size effects in bending are very important. A modification is made to the traditional weakest link theory for size effects to reflect the anisotropic nature of timber. Good agreement is obtained between tests and theory. The theory enables us to compare tests with different spans and (or) load configurations with an understanding not previously possible. Results from several large testing programs are used to quantify the theory. For bending members, length effects and load configuration effects are found to be much more important than depth effects, and a simple design method is proposed. Information for tension members is less comprehensive; nevertheless, a tentative suggestion for the design of those members is included.