We describe recent development of finite element error analysis. In finite element error analysis, estimating errors of Lagrange interpolation on triangles is very important. To obtain error estimations of Lagrange interpolation, it is believed that triangles or triangulations must satisfy a certain geometric conditions such as minimum or maximum angle conditions. The authors have found that the circumradius condition of triangles or triangulations is more essential than the minimum and maximum angle conditions. To show our claim, the authors extended techniques presented by Babuška-Aziz and Liu-Kikuchi. Our method can be applied to the case of Lagrange interpolation on tetrahedrons and Crouzeix-Raviart interpolation on triangles.