TWTs are widely used as high power microwave amplifiers for numerous defence, communication, and scientific applications. These devices are extremely complex in geometry and very expensive in manufacturing. Numerical modeling and simulation techniques are being used to improve the performance of these devices for higher output power, gain, efficiency, bandwidth and reliability even at very high frequencies. This paper presents details of the in-house developed software packages for the design and analysis of both helix and coupled-cavity TWTs. One software package, SUNRAY code, is described for large signal analysis of TWTs. Accuracies of these codes are ascertained by comparing simulated results with the experimental results and these codes are found highly accurate, self consistent and fast for interactive use. Recently, a space TWT of high efficiency, high gain, high linearity, small size and low weight is designed for a communication satellite system using in-house developed software packages.