ECONOMIC DEVELOPMENT THEORIES
The three building blocks of most growth models are: (1) the production function, (2) the saving function, and (3) the labor supply function (related to population growth). Together with a saving function, growth rate equals s/ß (s is the saving rate, and ß is the capital-output ratio). Assuming that the capital-output ratio is fixed by technology and does not change in the short run, growth rate is solely determined by the saving rate on the basis of whatever is saved will be invested.
Harrod-Domar Model

The Harrod-Domar theory delineates a functional economic relationship in which the growth rate of gross domestic product (g) depends directly on the national saving ratio (s) and inversely on the national capital/output ratio (k) so that it is written a g = s / k. The equation takes its name from a synthesis of analyses of growth process by two economists (Sir Roy Harrod of Britain and E.V. Domar of the USA). The Harrod-Domar model in the early postwar times was commonly used by developing countries in economic planning. With a target growth rate, the required saving rate is known. If the country is not capable of generating that level of saving, a justification or an excuse for borrowing from international agencies can be established. An example in the Asian context is to ascertain the relationship between high growth rates and high saving rates in the cases of Japan and China. It is more difficult to introduce the third building block of a growth model, the labor and population element. In the long run, growth rate is constrained by population growth and also by the rate of technological change.
Exogenous Growth model

The Exogenous Growth theory (or Neoclassical Growth Model) of Robert Solow and others places emphasis on the role of technological change. Unlike the Harrod-Domar model, the saving rate will only determine the level of income but not the rate of growth. The sources-of-growth measurement obtained from this model highlights the relative importance of capital accumulation (as in the Harrod-Domar model) and technological change (as in the Neoclassical model) in economic growth. The original Solow (1957) study showed that technological change accounted for almost 90 percent of U.S. economic growth in the late 19th and early 20th centuries. Empirical studies on developing countries have shown different results.
Even so, in our postindustrial economy, economic development, including in emerging countries is now more and more based on innovation and knowledge. Creating Porter's clusters is one of the strategies used. One well known example is Bangalore in India.
Surplus Labor Model

The Lewis-Ranis-Fei (LRF) theory of Surplus Labor is an economic development model and not an economic growth model.
Economic models such as Big Push, Unbalanced Growth, Take-off, and so forth, are only partial theories of economic growth that address specific issues. It is a model taking the peculiar economic situation in developing countries into account: unemployment and underemployment of resources (especially labor) and the dualistic economic structure (modern vs. traditional sectors). This model is a classical model because it uses the classical assumption of subsistence wage.
Here it is understood that the development process is triggered by the transfer of surplus labor in the traditional sector to the modern sector in which some significant economic activities have already begun. The modern sector entrepreneurs can continue to pay the transferred workers a subsistence wage because of the unlimited supply of labor from the traditional sector. The profits and hence investment in the modern sector will continue to rise and fuel further economic growth in the modern sector. This process will continue until the surplus labor in the traditional sector is used up, a situation in which the workers in the traditional sector would also be paid in accordance with their marginal product rather than subsistence wage.
The existence of surplus labor gives rise to continuous capital accumulation in the modern sector because (a) investment would not be eroded by rising wages as workers are continued to be paid subsistence wage, and (b) the average agricultural surplus (AAS) in the traditional sector will be channeled to the modern sector for even more supply of capital (e.g., new taxes imposed by the government or savings placed in banks by people in the traditional sector). In the LRF model, saving and investment are driving forces of economic development. This is in line with the Harrod-Domar model but in the context of less-developed countries. The importance of technological change would be reduced to enhancing productivity in the modern sector for even greater profitability and to promote productivity in the traditional sector so that more labor would be available for transfer.
Harris-Todaro Model

The Harris-Todaro (H-T) theory of rural-urban migration is usually studied in the context of employment and unemployment in developing countries. In the H-T model, the purpose is to explain the serious urban unemployment problem in developing countries. The applicability of this model depends on the development stage and economic success in the developing country.
The distinctive concept in the H-T model is that the rate of migration flow is determined by the difference between expected urban wages (not actual) and rural wages. The H-T model is applicable to less successful developing countries or to countries at the earlier stages of development. The policy implications are different from those of the LRF model. One implication in the H-T model is that job creation in the urban sector worsens the situation because more rural migration would thus be induced. In this context, China's policy of rural development and rural industrialization to deal with urban unemployment provides an example.

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"When God Want to Humiliate A Person then He Almighty Deprive him of Knowledge" Hazrat Ali A.S