1 Graphs.- 2 Closed Surfaces.- 3 Simplicial Complexes.- 4 HomoLogy Groups.- 5 The Question of Invariance.- 6 Some General Theorems.- 7 Two More General Theorems.- 8 Homology Modulo 2.- 9 Graphs In Surfaces.- Appendix: Abelian Groups.- Basic definitions.- Finitely generated (f.g.) and free abelian groups.- Quotient groups.- Exact sequences.- Direct sums and splitting.- Presentations.- Rank of a f.g. abelian group.- References.- List of Notation.