This report presents an overview of our recent near-field investigation of both local and surface plasmon resonance (SPR) with a scattering-type scanning near-field optical microscope (s-SNOM) which has sub-10 nanometer resolution. With the ability to perform near-field optical experiments at multiple excitation wavelengths simultaneously, this instrument has recorded near-field intensity and phase images of a wide range of subwavelength plasmonic structures: single nanohole and nanoslit, circular and elliptical hole arrays, etc. The near-field results obtained with different excitation wavelengths were confirmed by numerical calculation and were made direct correspondence with far-field observations by comprehensive models. The multi-wavelength s-SNOM proves to be an essential tool to unravel many interesting plasmonic phenomena in nanometer scale. This work investigates the nature of subwavelength plasmon optics which potentially will play an important role in the development of many innovative highly efficient optoelectronic devices (light-emitting devices and solar cells) and highly sensitive sensors based on SPR and surface-enhanced Raman scattering.