Since the invention of the laser in 1960, the relevant basic scientific research, technological development, and engineering application supported by lasers have developed rapidly, making optics enter a new laser era. In last two decades, with the development of laser technology and the growth of its application demand, a series of spatially structured light fields that have elaborate in-plane distributions of amplitude, phase, and polarization have been proposed, such as vector and vortex beams, self-healing and self-accelerating beams, which have shown great potential in solving bottleneck problems such as the diffraction limit of light wave. These spatially structured light fields have been successfully used in realms of super-resolution imaging, particle manipulation, laser micromachining, classical and quantum communication and information restoration, etc. On the other hand, the relevant theories and technologies of light field modulation have further promoted the development of other physical fields. With the in-depth research and wide application of spatially structured light field, scientists gradually focus on the three-dimensional structure control of light field. In this special perspective, the first problem to be solved is the control of light field in the longitudinal dimension, which will provide a more adequate means for the further development of the application of light field in optical micro-manipulation, microscopic imaging, optical processing, angular momentum control, information storage, three-dimensional control of photonic state and so on.In this paper, we give an overview of recent progress on light field intensity and polarization modulation in the longitudinal dimension, and discuss the resulted spatially structured light fields from spatial properties to application potentials. Firstly, we briefly review the on-axis intensity modulation of light field based on the Bessel spectrum control and coherent superposition principles, which enable to create longitudinally modulated fields with discrete and continuous intensity profiles. Next, we describe the light fields with arbitrary propagation trajectories along the longitudinal direction based on two basic theories, i.e., caustic theory and Bessel spectrum mapping theory, which lead to the nontrivial propagation that has with large bending angle in non-paraxial condition. Then, we introduce a special kind of light field with arbitrary trajectory, i.e., the light fields that spirally propagate around the propagating axis, followed by the discussion of generation methods and self-accelerating characteristics of light fields with equidistant spiral, non-equidistant spiral, segmented spiral, and radially structured profiles. We also place particular emphasis on the recent development of polarization conversion during optical beam propagating in free space, and reveal this intriguing phenomenon in the view of spin-orbital angular momentum coupling. Wherein, in contrast to the polarization conversion with intensity profile variation, we introduce a special longitudinal polarization manipulation that exist in scalar and vector beams with non-diffraction intensity profiles. In addition, we trace the joint control of light field intensity and the polarization structure under the tightly focusing condition, which induces a strong longitudinal field component to engineer the longitudinal distribution of light field, and summarize the special three-dimensionally structured light fields constructed from this method.Longitudinally controlling light field not only enriches the spatially structured light field and its relevant theory, e.g., the topologically structured vortex and vector light fields, which have topologies as a new degree of freedom in photonics, it is noteworthy that it also spawns numerous new photonic devices. However, there are still many opportunities and challenges in this rapidly developing research realm. At last, we envision the possible challenges and prospects of longitudinal modulation of light fields, such as how to realize the subtly longitudinal modulation with wavelength-scale variation period, the combined control of multiple parameters along the longitudinal direction, and the construction of novel eigenmodes with three-dimensional spatial correlation, which will stimulate broader and more in-depth theoretical and application exploration.