Thrombotic risk factors and thrombosis in children has been receiving increased attention. True idiopathic thrombosis is extremely rare in children. Most patients have a significant underlying medical condition and the presence of a central catheter is the most important risk factor. Children are more likely than adults to have one or more significant genetic abnormality or coagulation deficiency. This review discusses problems concerning the heterogeneity of thrombotic states in children and highlights the importance of understanding the concept of developmental hemostasis. Issues regarding step-wise test selection and the interpretation of results are addressed, as well as basic monitoring of anticoagulant drug effects.