In 1982, chronic myelomonocytic leukemia (CMML) was first classified in the category of myelodysplastic syndromes (MDSs), but it always seemed somewhat out of place compared with the rest of the MDS categories. In the 1990s, many argued that there were two different forms of CMML, a proliferative type and a myelodysplastic type. Then in 2001 the World Health Organization created a new category called the mixed myelodysplastic/myeloproliferative diseases, under which CMML was included. Although we still do not understand much about CMML pathogenesis nor do we have specific therapies for this disease, at least now most agree that it is in an appropriate category such that other areas of investigation can now proceed. On the other hand, we now understand a great deal of the pathogenesis underlying the disease now called juvenile myelomonocytic leukemia (JMML). JMML also fits in the new category of mixed myelodysplastic/myeloproliferative diseases. JMML is an excellent model malignancy for investigating and understanding dysregulated and aberrant signal transduction in the Ras pathway. It has also served as a teaching tool for exploring inherited predispositions to cancer.