Carbon NanoTubes (CNTs) are one of the most promising materials for future electronics. Among their possible applications, CNTFETs (Carbon NanoTube Field-Effect Transistors) are the most interesting since they are expected to sustain the transistor scalability, increasing performances of classical silicon-based devices as well. This book collects five chapters and is dedicated to the study of the design principles of analogue and digital circuits with CNTFETs, showing innovative aspects and highlighting the most original research results carried out by us in the recent years and developed at Electronic Devices Laboratory of Polytechnic University of Bari, Italy. In particular, in the first chapter, we will present a detailed study of Carbon NanoTubes (CNTs), introducing their physical characteristics which give them unique properties and enhanced versatility. The second chapter will be focused on an exhaustive description of the basic types of CNTFETs, analyzing for each one the principle of operation. In the third chapter we will examine the modelling issue, with particular reference to conventional CNTFETs, reviewing a compact, semi-empirical model of CNTFETs, already proposed by us. Here we will introduce new solutions to allow an easy implementation in the most common circuit simulators, such as SPICE and Verilog-A. The study of design principles of typical analogue circuits and logic blocks with CNTFETs will be described in the fourth chapter. We will discuss the simulation results obtained implementing the CNTFET model both in Verilog-A and in SPICE. A particular attention will be paid to the study of CNTFETs as memory devices through the design of a 16 bit SRAM. In the last chapter we will carry out static and dynamic analysis of basic digital circuits, verifying the results by means of a comparison with those obtained by Prof. Wong at Stanford University. We do believe that this book can be useful to graduate students and industrial researchers who are interested in learning about the recent progress and future challenges of this new and exciting research field.