This thesis presents observational kinematic results for a significant sample of disk galaxies. Spectra of both the gaseous and stellar components are available for almost the whole sample. The first aim was to make a comparative study of the gas and the stellar kinematics to see whether there was any correlation between the kinematic behavior and the morphological classification in disk galaxies. The study brought out a wide variety of kinematic phenomenology: gas rotating faster than stars, gas rotating at the same velocity as the stars, and sometimes gas rotating more slowly than the stars—also counterrotation of gas with respect to the stars and the increasingly frequently observed phenomenon of the counterrotation of stellar populations. In the central regions of the late‐type spirals we observe almost the same velocity gradient for the stars and the gas and also comparable velocity dispersions which are characterized by low values (∼50 km s−1) over the full observed radial range. We deduce that in most of the late‐type spirals we have studied the stars and the ionized gas are moving with essentially circular velocity. The central parts of early‐type disk galaxies reveal a wider variety of different behavior of stars and gas. It is normal to find that the velocity gradient of the stars with radius is less steep than that of the gas and that the velocity dispersion of the stars is higher than that of the gas (∼50 km s−1). This can be easily explained if the stellar and gas kinematics are dominated by dynamical pressure and by rotation, respectively. The observed stellar rotation can be corrected to that corresponding to circular velocity, as traced by the gas rotation, by including a term to account for asymmetric drift. We present also, for a subsample of galaxies where there is no evidence of triaxiality or of a bar, self‐consistent dynamical models to reproduce the available data, both spectroscopic and photometric. We show how the application of dynamical models is of value in understanding the kinematic structure of a galaxy, and we conclude that the morphological classification of a galaxy is not necessarily the best parameter to correlate with its kinematics. The models derive, using the stellar photometric data, the gravitational potential in which the ionized gas is expected to orbit. They take into account asymmetric drift, projection effects along the line of sight, and the non‐Gaussian shape of the line profiles due to the presence of different components with distinct dynamical behavior. The number of galaxies belonging to the sample is not large enough to draw completely general conclusions. However, we found a probable correlation between the presence of slowly rising gas rotation curves and the ratio of the bulge/disk half‐luminosity radii, whereas there is no obvious correlation with the parameter more normally used in morphological classification, i.e., the bulge/disk luminosity ratio. Systems with a diffuse dynamically hot component (bulge or lens) having a scale length comparable to that of the disk (e.g., NGC 772) are characterized by slowly rising gas rotation curves. On the other hand, in systems with a small bulge (e.g., NGC 5064, NGC 7782), the gas follows almost circular motions, regardless of the luminosity of the bulge itself. We point out similar behavior in the gas and stellar kinematics of some early‐type spiral galaxies modeled in a similar way by other authors.