Plasmonic nanostructures can enhance light-matter interactions in the subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional plasmonic devices are optimized to operate in a single wavelength band, which is not efficient for wavelength-multiplexed operations and quantum optical applications involving multi-photon nonlinear processes at multiple wavelength bands. Overcoming the limitations of single-resonant plasmonics requires development of plasmonic devices that can enhance the optical interactions at the same locations but at different resonance wavelengths. This dissertation comprehensively studies the theory, design, and applications of such devices, called "multiresonant plasmonic systems with spatial mode overlap". We start by a literature review to elucidate the importance of this topic as well as its current and potential applications. Then, we briefly discuss the fundamentals of plasmonic resonances and mode hybridization to thoroughly explore, classify, and compare the different architectures of the multiresonant plasmonic systems with spatial mode overlap. Also, we establish the black-box coupled mode theory to quantify the coupling of optical modes and analyze the complicated dynamics of optical interactions in multiresonant plasmonic systems. Next, we introduce the nanolaminate plasmonic crystals (NPCs), wafer-scale metamaterials structures that support many (>10) highly-excitable plasmonic modes with spatial overlap across the visible and near-infrared optical bands. The enabling factors behind the NPC's superior performance as multiresonant systems are also theoretically and experimentally investigated. After that, we experimentally demonstrate the NPCs application in simultaneous second harmonic generation and anti-Stokes photoluminescence (ASPL) with controllable nonlinear emission properties. By designing specific non-linear optical experiments and developing advanced ASPL models, this work addresses some important but previously unresolved questions on the ASPL mechanism as well. Finally, we conclude the dissertation by discussing the potential applications of out-of-plane plasmonic systems with spatial mode overlap in wavelength-multiplexed devices and presenting some preliminary results. / Doctor of Philosophy / Emergence of electronic devices such as cellphones and computers has revolutionized our lifestyles over the past century. By manipulating the flow/storage of electrons at the nanometer scale, electronic components can be very compact, but their speed and energy performance is ultimately limited due to ohmic losses and finite velocity of the electrons. In parallel, photonic devices and circuits have been proposed that by molding the flow of light can overcome the mentioned limitations but are not as integrable as their electronic counterparts. Plasmonics is an emerging research field that combines electronics and photonics using nanostructures that can couple the light waves to the free electrons in metals. By confining the light at deep subwavelength scales, plasmonic devices can highly enhance the light-matter interactions, with applications in ultrafast optical communications, energy-harvesting, optical sensing, and biodetection. Conventionally, plasmonic devices are optimized to operate with a single light color, which limits their performance in wavelength-multiplexed operations and ultrafast non-linear optics. For such applications, it is far more efficient to use the more advanced "multiresonant plasmonic systems with spatial mode overlap" that can enhance the optical interactions at the same locations but for multiple light colors. This dissertation comprehensively studies these systems in terms of the fundamental concepts, design ideas, and applications. Our work advances the plasmonic field from both science and technology perspectives. In particular, we explore and classify the strategies of building multiresonant plasmonic systems with spatial mode overlap for the first time. Also, we establish the black-box coupled mode theory, a novel framework for analysis and design of complicated plasmonic structures with optimized performance. Furthermore, we introduce the "nanolaminate plasmonic crystals" (NPCs), large area and cost-effective devices that can enhance the optical processes for both visible and near-infrared lights. Finally, we demonstrate NPCs ability in simultaneous frequency-doubling and broadband emission of light and come up with advanced theoretical models that can explain the light generation and color conversion in plasmonic devices.