A control engineer often describes a system through the use of block diagrams. In recent years, several computer aided engineering, CAE, tools have been developed to aid the engineer in drawing these diagrams. This paper will discuss an evaluation recently completed by the authors of three block diagram entry CAE tools . A simplified turbofan engine block diagram specification which was used to perform the evaluation is first discussed The paper then discusses some of the desired requirements for block types, connections, parameterization, and graphical manipulation. It is advocated that for direct engineering use the interface needs to be menu and cursor/ mouse driven. The characteristics and performance of three commerciaUy available block diagram computer aided engineering tools, Autocode, Model-C and Protoblock, are discussed relative to the desired requirements and control example. It is shown that Autocode and Model-C use basically the same approach though Autocode currently has a better library of block types. Protoblock uses an entirely different appoach which recieved a mixed evaluation.