Invasive fungal infections (IFIs) are on the increase not only among oncology and transplant patients but also among patients admitted to intensive care units (ICU). The rise in ICU IFIs can be attributed to the growing use of complex surgical procedures, invasive medical devices, and long-term, broad-spectrum antibiotic therapy. The majority of these life-threatening infections are caused by the well-known opportunistic pathogens Candida albicans and Aspergillus fumigatus, but new opportunistic pathogens, including yeast-like and other filamentous fungi, have emerged as additional causes. Invasive Candida infections, particularly candidemia, represent the most common IFI in critically ill patients. The species that cause candidemia markedly differ in their responses to antifungal drugs; for this reason, therapy must be tailored to the susceptibility characteristics of the infectious agent. Candidemia caused by non-albicans Candida species is increasing worldwide, and these infections are generally associated with high mortality rates, particularly bloodstream infections caused by C. krusei, which is innately resistant to fluconazole, or C. glabrata, which easily develops azole resistance. Although invasive yeast infections can be considered the most important causes of morbidity and mortality in ICU patients, pulmonary aspergillosis has recently emerged as an additional complication. Diagnosis of IFIs can be achieved using conventional approaches (microscopy, culture, and serology) and newer methods, including antigen detection and polymerase chain reaction (PCR) assays. Because most of the conventional approaches lack sensitivity, antigen detection and PCR assays could represent a valid alternative; however, these procedures need to be standardized and evaluated in a large number of patients.