Pyridine and pyrimidine compounds: or a pharmaceutically acceptable salt thereof, wherein m, n, R1, R2, R3, R4, R5, R6, R7, X1, X2, X3, X4, X5, X6, X7, X8, and Y are as defined in the specification; or a pharmaceutically acceptable salt thereof, wherein ring A, m, n, y, R2, R3, R4, R5, R6, R7, R8, R9, X1, X2, and ring A are as defined in the specification; and or a pharmaceutically acceptable salt thereof, wherein m, n, y, R2, R3, R4, R5, R6, R7, R9, X1, X2, and ring A are as defined in the specification; compositions containing them, and processes for preparing such compounds. Provided herein also are methods of treating disorders or diseases treatable by inhibition of PDE10, such as obesity, non-insulin dependent diabetes, schizophrenia, bipolar disorder, obsessive-compulsive disorder, and the like.