Schematic picture of sequential formation of molecular clouds by multiple compressions by overlapping dense shells driven by expanding bubbles. The thick red circles correspond to magnetized dense multiphase ISM where cold turbulent HI clouds are embedded in WNM. Molecular clouds can only be formed in the limited regions where the compressional direction is almost parallel to the local mean magnetic field lines or in regions experiencing an excessive number of compressions. An additional compression of a molecular cloud tends to create multiple filamentary molecular clouds. Once the line mass of a filament exceeds the critical value, even in a less massive molecular cloud, star formation starts. In general, star formation in a cloud accelerates with the growth in total mass of the cloud. Giant molecular clouds collide with one another at limited frequency. This produces very unstable molecular gas and may trigger very active star formation.