This chapter emphasizes pure acid toxicity with some reference to action of aluminum where appropriate. It discusses individual pollutants within the context of how they affect particular organs or physiological processes. The chapter begins with spawning and then considers the effects of acid (and often aluminum) on the different life stages. The reductions in pH of surface waters mobilize metals, especially aluminum. Atmospheric pollutants are deposited episodically such that there is usually an autumn and a spring decline in pH. The autumn decline coincides with the spawning and early embryonic development of species like lake trout, lake white fish, and lake herring. As embryonic development of fish eggs proceeds, sensitivity to acid changes. Hormones from the caudal neurosecretory system are also stimulated by acid waters. Changes in the level of these hormones are probably very much influenced by the degree of stress the fish is psychologically perceiving.