The goal of this research is to develop an understanding of the structure of pressure fluctuations in fluidized bed systems and how they characterize fluidized bed hydrodynamics. The use of fluctuation spectral analysis as a diagnostic and design tool for both bubbling fluidized beds (BFB) and circulating fluidized beds (CFB) is evaluated. Static pressure fluctuations are measured from two BFB models, two CFB models, and two Pyropower CFB boilers located at the ISU power plant/cogeneration facility. Analyzing the fluctuations using spectral analysis, shows that the structure of pressure fluctuations is governed by multiple phenomena. Similarities and differences between the nature of fluctuations in bubbling, turbulent, and fast fluidization regimes are discussed;The Bode plots of pressure fluctuations under bubbling/turbulent fluidization exhibit the characteristics of an oscillatory second order system. This system is governed by a combination of the natural frequency of the bed and surface phenomena. A theoretical model is developed to predict this natural frequency of oscillation in fluidized systems, and to explain the second order behavior. A comparison of pressure fluctuation behavior in two geometrically similar BFBs is conducted, which validates previously derived fluidized bed similitude parameters under most operating conditions;Multiple second order phenomena acting concurrently also govern CFB pressure fluctuations. Phenomena similar to that proposed for BFBs governs pressure fluctuations in the lower dense regions of the CFB. A wave phenomena is hypothesized to govern CFB fluctuations above the lower dense bed. Similar pressure dynamics and axial voidage profiles are not observed when currently proposed similitude parameters were matched in two CFBs. By replacing the dimensionless solids flux with a dimensionless riser loading parameter, this modified set of similitude parameters can be used to establish similar hydrodynamics in CFBs. Primarily due to the periodic operation of the coal feed system in the industrial scale CFB Boiler, pressure fluctuations analysis did not result in any definitive hydrodynamic information.