Phosphodiesterases (PDEs) are enzymes that play a crucial role in cellular signaling by breaking down cyclic nucleotides, such as cyclic AMP and cyclic GMP. These molecules are important for various physiological processes, including regulating blood flow, neurotransmission, and cell growth. By controlling the levels of cyclic nucleotides, PDEs help modulate the intensity and duration of signaling pathways. There are several types of phosphodiesterases, each with specific functions and tissue distributions. For example, PDE5 is primarily found in the smooth muscle of blood vessels and is targeted by medications like sildenafil to treat erectile dysfunction. Understanding PDEs is essential for developing therapies for various diseases, including cardiovascular and respiratory conditions.