Lymphatic Congenital Vascular Malformations (LCVM) represent a serious differential diagnosis for lymphedema. They are by no means tumors, but malformations of the vascular system. Vasculogenetic, extratruncular malformations must be distinguished from angiogenetic, truncular malformations; mixed forms are also found. The most important diagnostic methods are color-coded duplex ultrasound and thermography. Pure lymphatic malformations are only found in infancy. Lymphatic malformations can show volume fluctuations within short periods due to increased lymph production or resorption. In the diagnosis, in addition to the patient’s history, the clinical examination is crucial and goal-oriented. Instrumental investigations are reserved for fine diagnostics. Color-coded duplex sonography, thermography, MRI will help for the detection of findings not accessible by ultrasound in the head, thorax, retroperitoneum, or small pelvis. This vascular representation is also needed therapeutically after diagnosis and indication for embolization or sclerotherapy. A histology is required only if there is suspicion of a vascular tumor or another differential diagnosis. Rapamycin is also used in lymphatic malformations, but only shows efficacy in the extratruncular vasculogenetic forms, not in the truncular angiogenetic forms. It has therefore no effect on primary lymphoedema.