The work included in this dissertation addresses the complexities of underwater soundpropagation through fine-scale, 1-10 m, sound speed structures in the upper ocean. These small spatial variations in the ocean’s sound speed strongly affect sound waves at mid-frequencies (1-10 kHz) because the wavelengths of those sound waves (1.5-0.15) m approach the scale of these sound speed structures. Chapters 1 & 2 cover a collection of observations from a field experiment off the coast of San Diego that are used to analyze sound propagation through the measured sound speed profiles embedded with fine structures. Chapter 3 covers a set of oceanographic observations and a model of sound speed fluctuations driven by internal wave motion to analyze how fine-scale structures alter the power spectrum of oceanographic sound speed fluctuations. A synthesis of oceanographic and acoustic observations as well as models suggests that fine-scale structures cause additional acoustic propagation paths, caustics, and change the temporal spectrum of sound speed variability. The culmination of these effects explains why acoustic receptions at mid-frequencies propagating in the upper ocean are generally more variable than at lower frequencies in deeper parts of the ocean.