Himachal Pradesh came into being on 15th April, 1948 as a centrally
administered territory by the integration of 30 erstwhile princely States. At that time the State had four districts viz. Chamba, Mahasu, Mandi and Sirmaur .
Later in 1951, it became a part “C” State under a Governor with a 36 Member Legislative Assembly and a three member cabinet. In 1954, Bilaspur, another part ‘C’ State was merged with Himachal Pradesh thereby adding one more district with area of 1,168 sq. kms. And the strength of its Assembly was raised to 41.
In 1956, despite the majority recommendations of the State Re-organization Commission for its merger with Punjab, Himachal Pradesh retained its separate entity.
On November 1, 1956 it again became a Union Territory under an
Administrator designated as Lieutenant Governor and its Assembly was abolished. In 1960, a new border district of Kinnaur was carved out of Mahasu district. Then in
1963, Assembly was revived and a popular Ministry was formed. Till October, 1966 the old Himachal Pradesh comprised the six hill districts of Bilaspur, Chamba, Kinnaur, Mahasu, Mandi and Sirmaur with an area of 27,007 sq. kms. Having a population of 13, 51,144 persons (1951 Census).
On 1st November, 1966, it was enlarged by merging the district of Kangra, Shimla, Kullu, Lahaul-Spiti, the Nalagarh tehsil of Ambala district, some parts of Una tehsil of Hoshiarpur district and Dalhousie of Gurdaspur district of the then Punjab State. With this merger the total area of Himachal Pradesh increased to 55,673 sq. kms. And its population to 28,12,463(1961 Census). In 1966 Area of Himachal increased by 106% and Population by 108%.
On 18th December, 1970 the State of Himachal Pradesh Act was passed by Parliament and the new state came into being on 25th January, 1971. Thus H.P. emerged as the 18th state of Indian Union.
Reorganizations of the districts took place on 1st September, 1972 as a consequence where of two more new districts namely Una and Hamirpur were created mainly as a result of trifurcation of the erstwhile Kangra district. Also from the then existing districts of Mahasu and Shimla, new districts of Shimla and Solan were formed by reorganizing the boundaries of old districts.
Presently, the strength of Legislative Assembly of Himachal Pradesh is 68. To the Union Legislature, Himachal Pradesh is represented by 4 Members to Lok Sabha and 3 Members to Rajya Sabha.

Please Note

OLD Himachal includes: Chamba, Sirmaur, Kinnaur, Bilaspur, Mandi and Shimla.
New Himachal includes: Kangra, Kullu, Lahaul-Spiti, Hamirpur and Una
In Solan district: Solan and Arki belongs to old Himachal and Kasauli, Kandaghat and Nalagarh belongs to New Himachal.
According to census 1991: Old HP constitutes 48.4% of area and 52.3% of Population and New Himachal constitutes 51.6% of area and 47.7% of population.