Microbial signaling is the driver to initiate microbe-host and microbe-microbe interactions. When the host perceives a signal, it will somehow decide whether to activate innate immunity or mutualistic cooperation between the organism(s). However, decoding and understanding any coded signal is an enigma, especially if we do not know what organisms produce the signal and why they use it. Lipo-chitooligosaccharides are an example of a coded signal which are recognized as “symbiotic” communication signals produced by rhizobia and by two genetically different mycorrhizal fungi, Rhizophagus irregularis and Laccaria bicolor. Given the discovery of lipo-chitooligosaccharides being produced by such diverse fungi, here we tested the hypothesis that lipo-chitooligosaccharides are ubiquitous signal molecules among fungi. Using both biological assays (legume root hair branching and ENOD11 expression), and high-performance liquid chromatography coupled to mass spectrometry, we analyzed fungal exudates of 61 fungi across five phyla in the fungal kingdom and three species of oomycetes. We were able to detect lipo-chitooligosaccharides in fungal exudates in 55 fungi across the Fungal Kingdom with various lifestyles and growth habits but not in non-dimorphic yeasts (Saccharomyces cerevisiae and Candida glabrata) and oomycetes. Moreover, chitooligosaccharides were detected in all fungi and oomycetes tested. To further understand why non-symbiotic fungi would produce lipo-chitooligosaccharides, we tested the hypothesis that lipo-chitooligosaccharides is a quorum sensing molecule. We used specific chemically synthesized lipo-chitooligosaccharides (sulfated C16:0; non-sulfated C16:0; sulfated C18:1; and non-sulfated C18:1) and naturally produced chitooligosaccharides (CO4, CO5, and CO8). We were interested in answering the following questions: 1) Can exogenous lipo-chitooligosaccharides influence growth and development of a fungus that produces lipo-chitooligosaccharides? 2) Can exogenous lipo-chitooligosaccharides influence growth and development of a fungus that does not produce lipo-chitooligosaccharides? 3) What is the extent of growth and development affected by exogenous lipo-chitooligosaccharides in the Fungal Kingdom? To answer these questions, we tested these synthesized lipo-chitooligosaccharides against two ascomycetes; Aspergillus fumigatus, which produces lipo-chitooligosaccharides, and Candida glabrata, which did not have lipo-chitooligosaccharides detection. Moreover, we tested Rhodotorula mucilaginosa, which is a basidiomycete yeast, to determine if the lipo-chitooligosaccharides effect on fungi can be across phyla. Our results determined that exogenous lipo-chitooligosaccharides influence the behavior, growth, and development, and gene expression in fungi, regardless of their ability to produce lipo-chitooligosaccharides or not, and their phylogenic placement in the Fungal Kingdom. Finally, we wanted to test the hypothesis that the genes responsible for lipo-chitooligosaccharides production in fungi are homologous to the genes found in rhizobia. In the latter, the genes encoding for chitin synthase, chitin deacetylase, and acetyltransferase are well characterized and have been demonstrated to be responsible for lipo-chitooligosaccharides production based on gene knockout studies. We found two homologous chitin deacetylase genes in Aspergillus fumigatus Af293 strain that matched with rhizobia chitin deacetylase genes. We constructed a single and double chitin deacetylase knockout mutant to determine if chitin deacetylase encoding genes are indispensable for lipo-chitooligosaccharides production in fungi. Our results determined that the homologous chitin deacetylase genes are essential for lipo-chitooligosaccharides in fungi. In conclusion, results from this dissertation have determined that: 1) lipo-chitooligosaccharides are ubiquitous signal molecules found throughout the Fungal Kingdom, 2) lipo-chitooligosaccharides influence behavior, growth and development, and gene expression in fungi, and 3) rhizobia and fungi share homologous genes that are responsible for the production of lipo-chitooligosaccharides.