A linear-shaped mesoscale convective system (MCS) is one of the major forms of organized deep convection. A moving linear-shaped MCS is known as a squall line. This chapter focuses on how the development and intensity of linear-shaped MCSs are affected by environmental stability and humidity conditions. Numerical experiments with idealized settings indicated that the intensity of convective updrafts within simulated linear-shaped MCSs becomes weaker under tropical/oceanic conditions than under midlatitude/continental conditions. The dependence of the convective intensity on the free tropospheric humidity appears differently in environments with different temperature lapse rate. The analysis on the climatological characteristics of quasi-stationary MCSs in Japan, as a case of MCSs that develop in humid climate regions, indicated that middle-level relative humidity controls the development of such MCSs and also that linear-shaped MCSs preferably occur with a certain amount of vertical shear. High-resolution modeling of a linear-shaped MCS demonstrated that the horizontal resolution of about 100 m is able to capture the successive development of convective cells on the upstream side of a linear-shaped MCS and the stationarity of the MCS as a whole through representing more precisely a complex and complicated nature of topography and explicitly resolving convective motion.