An observational study of large-scale star formation processes in spiral galaxies with well-defined arm systems is presented. Combining new data on the current massive star formation (from H-alpha imaging) and on the total gas distribution (both atomic and molecular hydrogen, from H I and CO observations), the star formation efficiencies in- and outside the spiral arms are compared. For the grand-design spiral galaxy NGC~4321, new 21cm H I (VLA, New Mexico), J>CO (45-m telescope, Nobeyama, Japan) and H-alpha observations (4.2-m William Herschel Telescope, La Palma, Spain) were obtained, whereas for M51 data from the literature were used. The spatial resolution of all these data is high enough (typically 15") to distinguish arm from inter-arm emission. From an analysis of the massive star formation efficiency (MSFE) along the main arms of NGC~4321 and M51, and comparing with the neighboring inter-arm regions, it is found that the MSFE is higher in the arm regions than between the arms by an average factor of 2 to 3 in both galaxies. This enhancement is shown to be due only to enhanced star formation in the arms, and cannot be caused by e.g. dust extinction in the H-alpha, or by metallicity effects in the disks of the galaxies that may cause a change in the CO to H_2 conversion factor. This result is direct evidence for triggering of the star formation in the arms. In the case of M51, the arm/inter-arm ratio of the MSFE (called epsilon) shows a well-defined two-fold symmetric pattern between the two main arms, indicating that a global mechanism, probably a density wave system, organizes the star formation in the disc of that galaxy. In NGC 4321 such a pattern is absent, and epsilon is constant along the arms. The absence of a symmetric pattern in epsilon may be related to the presence of a bar in NGC 4321. Evidence for the existence of a bar in this galaxy is presented: a distortion in the H I velocity field, as well as the morphology of the galaxy in H-alpha and in the near-infrared. Such an anti-correlation between the presence of a bar and the presence of a symmetric pattern in the distribution of the star forming regions along the arms is seen among a small sample of other grand-design galaxies studied in H-alpha, although a statistical study is needed to generalize this finding. From along-the-arm profiles in H I, CO, and H-alpha, evidence is presented that in both M51 and NGC 4321 most of the atomic hydrogen in the star forming disk of the galaxy is formed by photo-dissociation of molecular hydrogen by young massive stars. From the 21 cm line study of NGC 4321, it is found that the H I disk of NGC 4321, in general limited to the extent of the optical disc, has a large though faint extension toward the SW, which may well have been caused by a close passage of the companion galaxy NGC 4322. A large-scale distortion of the H I velocity field is seen in the same direction. Other distortions in the velocity field due to the presence of a non-axisymmetric potential and of density wave streaming motions are also detected in H I. (SECTION: Dissertation Abstracts)