Lake Erie is once again experiencing intense cyanobacterial harmful algal blooms (CHABs) in the western basin.  While it is well known that cultural eutrophication is a primary driver of these phenomena, there is fierce debate over the roles of nitrogen (N) and phosphorus (P) in stimulating the growth and toxicity of CHABs. I will present the results from long-term monitoring, microcosm experiments and advanced genetic techniques.  Initial findings indicate indicate that while P is critical for initiating blooms, N may be more important for stimulating growth and toxin production.  Furthermore, CHABs that form in tributaries such as Sandusky Bay were primarily N limited throughout the bloom season.  Finally, metagenomic and metatranscriptomic results have shed light on why a non-N-fixing cyanobacterium (Planktothrix) can dominate in a chronically N limited system and reveal mechanisms that Microcystis can use to outcompete other phytoplankton during times of P limitation. Hence, both N and P must be considered when developing nutrient mitigation strategies.